arXiv subjects

C. Koen

Publications and source records attributed to C. Koen.

22 records · Page 2Linked to original sources

arXiv2000

Metallicity estimates for A-, F-, and G-type stars from the Edinburgh-Cape blue object survey

The Edinburgh-Cape Blue Object Survey is an ongoing project to identify and analyse a large sample of hot stars selected initially on the basis of photographic colours (down to a magnitude limit B~18.0) over the entire high-Galactic-latitude southern sky, then studied with broadband UBV photometry and medium-resolution spectroscopy. Due to unavoidable errors in the initial candidate selection, stars that are likely metal-deficient dwarfs and giants of the halo and thick-disk populations are inadvertently included, yet are of interest in their own right. In this paper we discuss a total of 206 candidate metal-deficient dwarfs, subgiants, giants, and horizontal-branch stars with photoelectric colours redder than (B-V)o = 0.3, and with available spectroscopy. Radial velocities, accurate to ~10-15 km/s, are presented for all of these stars. Spectroscopic metallicity estimates for these stars are obtained using a recently re-calibrated relation between Ca II K-line strength and (B-V)o colour. The identification of metal-poor stars from this colour-selection technique is remarkably efficient, and competitive with previous survey methods. An additional sample of 186 EC stars with photoelectric colours in the range -0.4 < (B-V)o < 0.3, comprised primarily of field horizontal-branch stars and other, higher-gravity, A- and B-type stars, is also analysed. Estimates of the physical parameters Teff, log g, and [Fe/H] are obtained for cooler members of this subsample, and a number of candidate RR Lyrae variables are identified.

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arXiv1998

KPD 0422+5421: A New Short Period Subdwarf B/White Dwarf Binary

The sdB star KPD 0422+5421 was discovered to be a single-lined spectroscopic binary with a period of P=0.0901795 +/- (3\times 10^{-7}) days (2 hours, 10 minutes). The U and B light curves display an ellipsoidal modulation with amplitudes of about 0.02 magnitudes. The sdB star contributes nearly all of the observed flux. This and the absence of any reflection effect suggest that the unseen companion star is small (i.e. R_comp ~ 0.01 solar radii) and therefore degenerate. We modeled the U and B light curves and derived i = 78.05 +/- 0.50 degrees and a mass ratio of q = M_comp/M_sdB = 0.87 +/- 0.15. The sdB star fills 69% of its Roche lobe. These quantities may be combined with the mass function of the companion (f(M) = 0.126 +/- 0.028 solar masses) to derive M_sdB = 0.72 +/- 0.26 solar masses and M_comp = 0.62 +/- 0.18 solar masses. We used model spectra to derive the effective temperature, surface gravity, and helium abundance of the sdB star. We found T_eff = 25,000 +/- 1500K, log g = 5.4 +/- 0.1, and [He/H] = -1.0. With a period of 2 hours and 10 minutes, KPD 0422+5421 has one of the shortest known orbital periods of a detached binary. This system is also one of only a few known binaries which contain a subdwarf B star and a white dwarf. Thus KPD 0422+5421 represents a relatively unobserved, and short-lived, stage of binary star evolution.

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arXiv1997

Discovery and analysis of Gamma Doradus type pulsations in the Cloud and the effect of metallicity on pulsation

We present multi-site photometric observations of the F0 IV star HR 2740=QW Pup which reveal it to be a Gamma Dor type variable pulsating with four frequencies: 1.0434, 0.9951, 1.1088, 0.9019 c\d. These data were obtained at the European Southern Observatory and South African Astronomical Observatory over a time baseline spanning from 1997 January 14 to 1997 February 11. The 1.0434 c\d term dominates in amplitude (10 mmag) over the other three (each less than 5 mmag); the light curve comprising these four frequencies seems to be very stable and no residual power is left in the power spectrum. During the analysis particular attention was paid to methodological aspects, which cannot be neglected considering the proximity of the frequencies to 1 c\d. Physical parameters were also derived for all the well-known Gamma Dor stars, confirming that this class is very homogenous. In the framework of the campaign, two Ap stars (OU Pup=HR 2746 and PR Pup=HR 2761) were also observed. The photometric differences between these rotating variables and HR 2740 are emphasized, corroborating the pulsational nature of the Gamma Dor stars. It is further demonstrated that the rotational splitting cannot be a suitable explanation of the observed frequency content of HR 2740.

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arXiv1996

The Gamma Doradus variables, a new class of pulsating stars. The case of HD 224945

A multisite campaign involving five observatories allowed us to solve the light curve of the Gamma Doradus star HD 224945. The multiperiodic content, the high frequency term at 3.00 c/d and the frequency spacing between the observed terms strongly support its pulsational nature. Considering the low frequency terms observed in Gamma Dor stars, it seems that g--mode pulsators can be found near the cold border of the instability strip.

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