Browsing by Subject "atmospheric"

Now showing 1 - 3 of 3

Carbon And Oxygen Abundances Across The Hertzsprung Gap
(2014-08) Adamczak, Jens; Lambert, David L.; Adamczak, Jens; Lambert, David L.
We derived atmospheric parameters and spectroscopic abundances for C and O for a large sample of stars located in the Hertzsprung gap in the Hertzsprung-Russell diagram in order to detect chemical peculiarities and get a comprehensive overview of the population of stars in this evolutionary state. We have observed and analyzed high-resolution spectra (R = 60,000) of 188 stars in the mass range 2-5M(circle dot) with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory including 28 stars previously identified as Am/Ap stars. We find that the C and O abundances of the majority of stars in the Hertzsprung gap are in accordance with abundances derived for local lower-mass dwarfs but detect expected peculiarities for the Am/Ap stars. The C and O abundances of stars with T-eff < 6500 K are slightly lower than for the hotter objects but the C/O ratio is constant in the analyzed temperature domain. No indication of an alteration of the C and O abundances of the stars by mixing during the evolution across the Hertzsprung gap could be found before the homogenization of their atmospheres by the first dredge-up.
The Discovery Of Stellar Oscillations In The K Giant Iota Draconis
(2008-11) Zechmeister, M.; Reffert, S.; Hatzes, A. P.; Endl, M.; Quirrenbach, A.; Endl, M.
iota Dra (HIP 75458) is a well-known example of a K giant hosting a substellar companion. We present radial velocity measurements of this star from observations taken with three different instruments spanning nearly 8 years. They show more clearly that the RV period is long-lived and coherent thus supporting the companion hypothesis. The longer time baseline now allows for a more accurate determination of the orbit with a revised period of P = 511 d and an additional small linear trend, indicative of another companion in a wide orbit. Moreover we show that the star exhibits low amplitude, solar-like oscillations with frequencies around 3-4 d(-1) (34.7-46.3 mu Hz).
A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs. DA Stars
(2010-03) Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.; Williams, Kurtis A.; Falcon, Ross E.; Winget, D. E.; Montgomery, Michael H.; Williams, Kurtis A.
We measure apparent velocities (v(app)) of the H alpha and H beta Balmer line cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Assuming these WDs are nearby and comoving, we correct our velocities to the local standard of rest so that the remaining stellar motions are random. By averaging over the sample, we are left with the mean gravitational redshift, < v(g)>: we find < v(g)> = < v(app)> = 32.57 +/- 1.17 km s(-1). Using the mass-radius relation from evolutionary models, this translates to a mean mass of 0.647(-0.014)(+0.013)M(circle dot). We interpret this as the mean mass for all DAs. Our results are in agreement with previous gravitational redshift studies but are significantly higher than all previous spectroscopic determinations except the recent findings of Tremblay & Bergeron. Since the gravitational redshift method is independent of surface gravity from atmosphere models, we investigate the mean mass of DAs with spectroscopic T(eff) both above and below 12,000 K; fits to line profiles give a rapid increase in the mean mass with decreasing T(eff). Our results are consistent with no significant change in mean mass: < M >(hot) = 0.640 +/- 0.014 M(circle dot) and < M >(cool) = 0.686(-0.039)(+0.035)M(circle dot).