New results from COS observations of solar-mass stars and premain sequence stars
Jeffrey Linsky (JILA, University of Colorado, NIST)
Thursday 19th May 2011, 11:00 am
Classroom (A-101), 60 Garden Street
The high sensitivity and very low detector background of the
Cosmic Origin Spectrograph (COS) on HST allows us to detect spectral
features that could not be studied on stars with previous ultraviolet
instruments. I mention two new results that we have obtained with COS.
In our study of solar-mass stars with very different rotational periods
(and ages), we now measure the far-UV continuum emission formed in the
stellar chromosphere. Although measurements of the solar far-UV continuum
have been used to determine the thermal structure of solar chromosphere
features, similar observations of stars have not been feasible before COS.
We show that the range of far-UV brightness temperatures seen on solar-mass
stars between slow and rapid rotators is similar to the range of brightness
temperatures seen on the Sun between dark and bright features.
Molecular hydrogen fluorescent emission pumped by Lyman-alpha is typically
seen in the UV spectra of pre-main sequences stars, but no direct evidence
for the pumping process has been seen previously. We now find weak absorption
features against the Lyman-alpha emission line at the wavelengths predicted
for the pumping transitions. For many of the pumping transitions, the
absorbed energy equals the corresponding fluorescent emission, but for some
pumping transitions there is an additional absorber along the line of sight.
We identify the absorber as Doppler-shifted neutral hydrogen, which provides
a new diagnostic of accretion.