The darkest bright star:: Chandra X-ray observations of Vega

We present X-ray observations of Vega obtained with the Chandra High Resolution Camera and Advanced CCD Imaging Spectrometer. After a total of 29 ks of observation with Chandra, X-rays from Vega remain undetected. We derive upper limits to the X-ray luminosity of Vega as a function of temperature over the range of 10(5) - 10(7) K and find a 99.7% upper limit as low as similar to 2 x 10(25) ergs s(-1) at T = 10(6.2) K. We also compare these new deeper observations with the limit derived from a reanalysis of ROSAT PSPC data. Our X-ray luminosity limit for Vega is still greater than predictions of post-Herbig Ae phase X-rays from the shear dynamo model proposed by Tout & Pringle for a Vega age of 350 Myr. If the age of Vega is closer to 100 Myr, as suggested by some indicators, our X-ray limit is then similar to Tout-Pringle model predictions. Current X-ray observations of Vega are therefore unable to discriminate between different scenarios explaining the X-ray activity of the convectively stable Herbig Ae/Be stars. Further progress is more likely to be achieved through X-ray observations of younger main-sequence early-type A stars, whose conjectured residual post-Herbig Ae phase X-ray activity would be significantly higher.