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On-Going Instrumentation: High Stability Lab Spectrograph

The High Stability Lab in the telescope pier provides a stable environment for precision radial velocity measurements. A fiber-fed, bench-mounted optical spectrograph with R=120,000, usable with an absorption cell in 500-600 nm range, would provide the greatest sensitivity currently for detection of low mass companions. A higher resolution mode, R>300,000 over the 380 to 1000 nm range, would enable many studies in stellar physics, including convection and magnetic fields, and also sufficient resolution for detailed studies of the ISM.

Performance Guidelines for the High Stability Lab Spectrograph:

Science Illustrations:

For a wide range of problems in the physics of stellar atmospheres, stellar line profiles must be resolved which, for late-type stars, demands <1 km/s or R> 300,000, at optical and near-IR wavelengths. Observations covering a range of spectral types would also allow a realistic theory for the changes that depend on evolution, and differences in mass. Convective motions can be estimated from asymmetries in line bisectors derived from high-resolution spectra. These would provide new insights into turbulence and dredge up rates providing important data for the interpretation of surface abundances and for understanding the importance of rotation in mixing. For studies of infalling cometary bodies in post-planet-building disks, e.g. the beta-Pic disk, a resolution of R=120K would be adequate for initial studies, with R~300K-500K for detailed work.

For stars with weak integrated magnetic fields like the Sun, Zeeman splitting cannot be resolved. On the other hand, stars with strong fields such as active M-dwarfs are generally too faint for observation with adequate spectral resolution. Thus, results to date are inconclusive. In particular the question of field strength vs. area coverage is ambiguous since it requires detailed modeling of the line profiles, particularly of many lines with different temperature sensitivities and continuum contrasts.

Abundances and luminosities of supergiants measured at R~120,000 in the nearest galaxies will impose limitations on estimates of fundamental stellar properties in different environments, particularly the Magellanic Clouds where accurate abundances and calibrated mass-loss rates as a function of luminosity can be determined. With Gemini, this technique could be extended to nearby galaxy groups.

A good way to disentangle dust depletion and abundances at the highest resdshifts is to measure FeII, which has a range of lines from 234-260nm, and the MgII doublet at 280nm. For redshifts above 2.7 the MgII lines are shifted into the infrared, and for z > 3.3 all are. At the highest redshifts available so far, z=4.39, the MgII doublet is in the infrared H window and a number of the FeII lines fall in the J band. To unscramble effects of ISM gas temperature at high redshifts from any turbulent motions requires resolving the lines at about the level of the narrower ones, so R=100,000 or more is indicated. If these temperatures can be determined they provide a direct comparison with local conditions, and constraints on the heating/cooling at high redshifts. Knowledge of the true line widths also helps firm up abundance estimates.

The case for spectral resolution R=300,000 for quasar absorption has a common thread with that for the R=120,000, to resolve the lines and so measure temperatures and abundances. There is an expectation that there will be cold clouds at high redshifts because stars must have formed then, and there are already indicators in one or two cases of cold material in quasar absorption systems.


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Last update June 5, 1998; Ruth A. Kneale