It may not be that the oblateness of many stars is sufficient to be measured. But if a useful relationship could be derived then spectra from space telescopes (Gaia?) might provide a means of estimating the oblateness of fast spinning stars (would Gaia be able to provide measurements of oblateness for nearby stars by virtue of its resolving power?).
I presume there are physical models already that relate star spin rate (measured from blurring of emission lines?) to oblateness but to my naive view it looks like direct oblateness measurements may be much less available. Perhaps this could be a way of identifying populations of very fast spinning stars?
Maybe I should email a query to the Gaia people - give them something else to do with their data that doesnt appear to be on the current TTD list?
Gaia is not
a large telescope (and it does not
do very high resolution spectroscopy).
Spinning Gaia sees many many multiple
blurred images of each individual star.
By statistically combining those many multiple
Gaia can very accurately pinpoint the central position
of each star
but it was never designed to actually resolve those multiple blurred images.
Thanks for the interesting background on Achernar. As to your response to my response perhaps you could comment on the following:
1. Gaia I agree is not a telescope in the old fashion sense of a Galiliean tube. But saying it is not a telescope in the modern sense seems strange - at bit like saying a radio telescope is not a telescope because it receives a complex signal that needs to be electronically processed to obtain a picture like photograph. To use the definition in Wiki:
"an optical instrument that aids in the observation of remote objects by collecting electromagnetic radiation"
Gaia certainly fits this description at least. e.g. http://sci.esa.int/gaia/59023-gaia-s-sn ... er-galaxy/
Its way of assembling images is 'unconventional'. But then most of the spectacular 'telescope' pictures we get these days are fitered ensembles and artefacts to varying degrees.
Perhaps you could clarify what the definition of 'telescope' is - say by a link to generally accepted authority say the IAU noting that 'Gaia' itself is also refere to the space observatory.
2. Gaia may not do high resolution spectroscopy across the spectrum, but:
" The Radial-Velocity Spectrometer (RVS) is used to determine the velocity of celestial objects along the line of sight by acquiring high-resolution spectra in the spectral band 847–874 nm (field lines of calcium ion) for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s (V=11.5) and 30 km/s (V=17.5). The measurements of radial velocities are important to correct for perspective acceleration which is induced by the motion along the line of sight." The RVS reveals the velocity of the star along the line of sight of Gaia by measuring the Doppler shift of absorption lines in a high-resolution spectrum." (Wiki)
And it does respectable 'synthetic' spectroscopy with a resolution of 10 nm give or take "The passbands are derived by the convolution of the response curves of the optics and the QE curves of the CCDs" - Jordi, C., Gebran, M., Carrasco, J., de Bruijne, J., Voss, H., Fabricius, C., . . . Mora, A. (2010). Gaia broad band photometry. Astronomy & Astrophysics, 523, A48. which I wonder might be sufficient to look at black body radiation changes.
3. Gaia complements high resolution spectroscopy enormously - Gilmore, G., Randich, S., Asplund, M., Binney, J., Bonifacio, P., Drew, J., . . . Micela, G. (2012). The Gaia-ESO public spectroscopic survey. The Messenger, 147, 25-31. section "Why not just wait for Gaia".
To my untutored self this suggests:
a. Gaia should detect lots of slow v. fast rotating stars based on spectroscopy and the broadenning of the Calcium lines, identify stars which are comparable (e.g. similar metalicity and emission peak) but differ in their rotation, and concurrently provide spectra from which you could provide a first cut on how spectra change the faster a star rotates using an enormous sample of comparable stars small and large, which could then be combined with more detailed data such as that from Gaia-ESO.
b. Gaia does not provide high resolution spectra in the strict single instrument sense. But these references suggest it will be central to transforming stellar spectroscopy way beyond its current state by providing statistical power that seems to come when you combine different measurement methods.