Re:Exoplanet Radial Velocity

21 November 2012 at 4:11 pm #576138

Participant

Posted by Robin Leadbeater at 16:11 on 2012 Nov 21

TonyAngel wrote:

Under this topic Radial Velocity will be discussed.

Was:exoplanet transitsTonyAngel wrote:

Thank you Robin. Yes I had read Christian Buil’s page. As I already had the use of the SGS I was hoping that I could at least see if it could have spotted the larger doppler shift exoplanents, or failing that spectroscopic binaries just to do a proof of concept before seeing if a more expensive spectrograph could be justified. Perhaps I can just use the SGS for confirmation of exoplanet’s star spectrum, (allowing this could be contamininated by the planet). As you can guess I am rather new to spectroscopy from a practical angle.

SBig claim an RV precision of +-6km/s with a 600l/mm grating for the SGS which is probably realistic (I can get down to +-1km/s with my LHIRES III under favourable conditions but that is with a 2400l/mm grating which gives ~10 higher dispersion. Entry level to just detect the exoplanet round Tau Boo which I think is the easiest is ~200m/s and Buil’s precision was around +-50m/s (Which is just 0.001A at 6000A !) so quite a tough target.Actually measuring binary star component RV can be tricky as the two spectra blend and can be difficult to separate. Here is a nice easy example, Beta Aurigae using the LHIRES at 0.3A resolution.http://www.threehillsobservatory.co.uk/astro/spectra_35.htmExoplanet measurements are more straightforward (though not easier!) as there is only one component (The contribution from the planet either from planet temperature, reflected starlight or absorption during transit due to an atmopshere can be safely ignored at visible wavelengths at least. This is the task of a future generation of exoplanet hunting space telscopes, looking for signs of life) I would say a good way of measuring the capabilities of a proposed system would be to repeatedly measure the RV of a radial velocity standard for several nights and see how much scatter you get. Then you can start beating down the variability until you are at least well below +-1km/s. You would then be ready to try for an exoplanet.CheersRobin