46 research outputs found
THuCIDIDES: a high-efficiency multimode spectrograph design for the Hale Telescope
This paper describes the operating parameters and initial design of a new spectrograph proposed for the 200-inch Hale Telescope at Palomar Observatory. The instrument, whose working name is THuCIDIDES (Two Hundred-inch Cassegrain Image- Deblurred Interchangeable-Disperser/Echelle Spectrograph), will feature high system efficiency and multiple modes of operation, including low- and intermediate-resolution long slit and multi-slit capability over 12.5 X 3 arcmin fields, and a cross-dispersed echelle mode covering 3800 - 8500 angstrom at R equals 20,000 (with a 1.2 arcsecond slit) up to R equals 60,000 (with an image slicer). A 4096 X 4096 pixel CCD will serve as the detector. The quasi-Littrow echelle configuration and use of a prism cross-disperser will result in high system efficiency, estimated at approximately equals 14%. The compact design will permit mounting in the Cassegrain ring plane, to reduce susceptibility to flexure. An optional fast-guiding tilt mirror provides modest improvement to seeing FWHM and slit throughput
THuCIDIDES: a high-efficiency multimode spectrograph design for the Hale Telescope
This paper describes the operating parameters and initial design of a new spectrograph proposed for the 200-inch Hale Telescope at Palomar Observatory. The instrument, whose working name is THuCIDIDES (Two Hundred-inch Cassegrain Image- Deblurred Interchangeable-Disperser/Echelle Spectrograph), will feature high system efficiency and multiple modes of operation, including low- and intermediate-resolution long slit and multi-slit capability over 12.5 X 3 arcmin fields, and a cross-dispersed echelle mode covering 3800 - 8500 angstrom at R equals 20,000 (with a 1.2 arcsecond slit) up to R equals 60,000 (with an image slicer). A 4096 X 4096 pixel CCD will serve as the detector. The quasi-Littrow echelle configuration and use of a prism cross-disperser will result in high system efficiency, estimated at approximately equals 14%. The compact design will permit mounting in the Cassegrain ring plane, to reduce susceptibility to flexure. An optional fast-guiding tilt mirror provides modest improvement to seeing FWHM and slit throughput
Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13
High-resolution optical spectra of thirteen blue horizontal-branch (BHB)
stars in the globular cluster M13 show enormous deviations in element
abundances from the expected cluster metallicity. In the hotter stars (T_eff >
12000 K), helium is depleted by factors of 10 to 100 below solar, while iron is
enhanced to three times the solar abundance, two orders of magnitude above the
canonical metallicity [Fe/H] ~= -1.5 dex for this globular cluster. Nitrogen,
phosphorus, and chromium exhibit even more pronounced enhancements, and other
metals are also mildly overabundant, with the exception of magnesium, which
stays very near the expected cluster metallicity. These photospheric anomalies
are most likely due to diffusion --- gravitational settling of helium, and
radiative levitation of the other elements --- in the stable radiative
atmospheres of these hot stars. The effects of these mechanisms may have some
impact on the photometric morphology of the cluster's horizontal branch and on
estimates of its age and distance.Comment: 11 pages, 1 Postscript figure, uses aaspp4.sty, accepted for
publication in ApJ Letter
Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15
High-resolution optical spectra of eighteen blue horizontal-branch (BHB)
stars in the globular cluster M15 indicate that their stellar rotation rates
and photospheric compositions vary strongly as a function of effective
temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal
abundances are in rough agreement with the canonical cluster metallicity, and
the v sin i rotations appear to have a bimodal distribution, with eight stars
at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at
Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron
and titanium are enhanced by a factor of 300 to solar abundance levels.
Magnesium maintains a nearly constant abundance over the entire range of Teff,
and helium is depleted by factors of 10 to 30 in three of the hotter stars.
Diffusion effects in the stellar atmospheres are the most likely explanation
for these large differences in composition. Our results are qualitatively very
similar to those previously reported for M13 and NGC 6752, but with even larger
enhancement amplitudes, presumably due to the increased efficiency of radiative
levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar
rotation explicitly preventing the onset of the diffusion mechanisms among a
subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ
Abundances in Stars from the Red Giant Branch Tip to the Near the Main Sequence in M71: I. Sample Selection, Observing Strategy and Stellar Parameters
We present the sample for an abundance analysis of 25 members of M71 with
luminosities ranging from the red giant branch tip to the upper main sequence.
The spectra are of high dispersion and of high precision. We describe the
observing strategy and determine the stellar parameters for the sample stars
using both broad band colors and fits of H profiles. The derived
stellar parameters agree with those from the Yale stellar evolutionary
tracks to within 50 -- 100K for a fixed log g, which is within the level of the
uncertainties.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 1 of a pair
Stellar Astrophysics with a Dispersed Fourier Transform Spectrograph. II. Orbits of Double-lined Spectroscopic Binaries
We present orbital parameters for six double-lined spectroscopic binaries
(iota Pegasi, omega Draconis, 12 Bootis, V1143 Cygni, beta Aurigae, and Mizar
A) and two double-lined triple star systems (kappa Pegasi and eta Virginis).
The orbital fits are based upon high-precision radial velocity observations
made with a dispersed Fourier Transform Spectrograph, or dFTS, a new instrument
which combines interferometric and dispersive elements. For some of the
double-lined binaries with known inclination angles, the quality of our RV data
permits us to determine the masses M_1 and M_2 of the stellar components with
relative errors as small as 0.2%.Comment: 41 pages, 8 figures, accepted by A
An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553
We provide a high dispersion line-by-line abundance analysis of five red HB
stars in the extremely metal rich galactic globular cluster NGC 6553. These red
HB stars are significantly hotter than the very cool stars near the tip of the
giant branch in such a metal rich globular cluster and hence their spectra are
much more amenable to an abundance analysis than would be the case for red
giants.
We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the
mean abundance in the galactic bulge found by McWilliam & Rich (1994) and
considerably higher than that obtained from an analysis of two red giants in
this cluster by Barbuy etal (1999). The relative abundance for the best
determined alpha process element (Ca) indicates an excess of alpha process
elements of about a factor of two. The metallicity of NGC 6553 reaches the
average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap
A New Spin on the Problem of HorizontalâBranch Gaps: Stellar Rotation along the Blue Horizontal Branch of Globular Cluster M13
We have determined the projected rotational velocities of 13 blue horizontal-branch (BHB) stars in the globular cluster M13 via rotational broadening of metal absorption lines. Our sample spans the photometric gap observed in the horizontal-branch distribution at T_(eff) 11,000 K and reveals a pronounced difference in stellar rotation on either side of this featureâblueward of the gap, all the stars show modest rotations, v sin i < 10 km s^(-1), while to the red side of the gap, we confirm the more rapidly rotating population (v sin i â 40 km s^(-1)) previously observed by R. C. Peterson and coworkers. Taken together with these prior results, our measurements indicate that a star's rotation is indeed related to its location along the HB, although the mechanism behind this correlation remains unknown. We explore possible connections between stellar rotation and mass-loss mechanisms which influence the photometric morphology of globular cluster HBs
The Masses and Evolutionary State of the Stars in the Dwarf Nova SS Cygni
The dwarf nova SS Cygni is a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg with the Hobby-Eberly Telescope (HET). Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K_{K} = 162.5 +/- 1.0 km/s and q= M_{K} /M_{wd} = 0.685 +/- 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 +/- 0.075 of the total flux at 5500 angstroms. Taking the weighted average of our results with previously published results obtained using similar techniques, we find = 163.7 +/- 0.7 km/s and = 0.683 +/- 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45 deg. <= i <= 56 deg. The derived masses of the K star and white dwarf are M_{K} = 0.55 +/- 0.13 M_sun and M_{wd} = 0.81 +/- 0.19 M_sun, where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main-Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted
Stellar Astrophysics with a Dispersed Fourier Transform Spectrograph. I. Instrument Description and Orbits of Single-lined Spectroscopic Binaries
We have designed and constructed a second-generation version of the Dispersed
Fourier Transform Spectrograph, or dFTS. This instrument combines a spectral
interferometer with a dispersive spectrograph to provide high-accuracy,
high-resolution optical spectra of stellar targets. The new version, dFTS2, is
based upon the design of our prototype, with several modifications to improve
the system throughput and performance. We deployed dFTS2 to the Steward
Observatory 2.3-meter Bok Telescope from June 2007 to June 2008, and undertook
an observing program on spectroscopic binary stars, with the goal of
constraining the velocity amplitude K of the binary orbits with 0.1% accuracy,
a significant improvement over most of the orbits reported in the literature.
We present results for radial velocity reference stars and orbit solutions for
single-lined spectroscopic binaries.Comment: accepted by Ap