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Spectral energy distributions of white dwarfs in 47 Tucanae

  1. Nov 23, 2014 #1
    "We present a new distance determination to the Galactic globular cluster 47 Tucanae by fitting the spectral energy distributions of its white dwarfs (WDs) to pure hydrogen atmosphere WD models. Our photometric data set is obtained from a 121-orbit Hubble Space Telescope program using the Wide Field Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images. These images cover more than 60 arcmin2 and extend over a radial range of 5-13.7 arcmin (6.5-17.9?pc) within the globular cluster. Using a likelihood analysis, we obtain a best-fitting unreddened distance modulus of (m ? M)o = 13.36 ? 0.02 ? 0.06 corresponding to a distance of 4.69 ? 0.04 ? 0.13?kpc, where the first error is random and the second is systematic. We also search the WD photometry for infrared excess in the F160W filter, indicative of low-mass companions, and find no convincing cases within our sample."

    hi guys, so this taken from http://iopscience.iop.org/1538-3881/143/2/50 and I'm trying to decipher what it means. I love astronomy, but I'm no astrophysicist so most of this going right over my head. Are they saying that they discovered that 47 tucanae is at a different distance than what was previously believed? I could really use some help here I'm going to meet Michael Shara in a few days and I want to be able to ask him questions about the research that he performed...

    If someone could just take a quick look at that above paragraph and maybe the introduction and tell if I'm on the right track or not it would greatly appreciated.
  2. jcsd
  3. Nov 23, 2014 #2


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    I wouldn't say that they discovered that 47 tucanae is at a different distance, but that their measurement gives a different distance than other measurements. There are several different ways of measuring distances, and each one will give you slightly different ranges.
  4. Nov 23, 2014 #3

    Ken G

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    The idea is, you want to know the age of globular clusters, because they tell the story of the early formation of our galaxy, but you want to know which chapter to place them in the chronological story. To get the age of a globular cluster, you often look at the white dwarfs, because their evolution responds to age. But you need to know the distance to the cluster. That's also true if you use main-sequence stars to get the age, because to get the turn-off of the main sequence, you need to know the distance so you know the luminosity at the turn-off. So their goal is to get distances, by using models of white dwarf spectra, which they feel they can model very accurately. Then they just include the degree of dust reddening to get the distance. I don't know anything about the uncertainties, but they expect to have very good accuracy and precision, better than previous approaches. Whether or not the inferred age changes as a result of their analysis, they have better confidence in their result.
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