Current spectroscopic techniques yield Doppler-shift errors of 10 to 50 ms -1 , barely adequate to detect reflex velocities caused by Jupiter-like and lower-mass planets. We describe a technique which yields relative radial-velocity errors of 3 ms -1 . This technique makes use of a fast echelle spectrograph at resolution of R=62,000 and a large-format CCD which acquires the entire visible and near-IR spectrum in each exposure. Starlight is sent through an iodine absorption cell placed at the spectrometer entrance slit. The resulting superimposed iodine lines provide a fiducial wavelength scale against which to measure radial-velocity shifts. The shapes of iodine lines convey the PSF of the spectrometer to account for changes in spectrometer optics and illumination on all time scales. We construct a model of each observed spectrum by multiplying a stellar spectrum with an iodine spectrum and convolving the result with the spectrometer PSF. The free parameters of the model include the wavelength scale, spectrometer PSF, and stellar Doppler shift. All model parameters are derived anew for each exposure and the synthesis is done on a grid of CCD sub-pixels, using spline functions as interpolation predictors. We present Doppler tests of the Sun, rCeti, and 107 Psc, observed with the Lick and Keck echelles. All exhibit apparent errors of about 3 ms -1 , maintained on time scales of minutes to a year. This precision agrees with the theoretically predicted errors that stem primarily from photon statistics.

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The extremely young cluster IC 348 has been monitored photometrically over five observing seasons from 1998 December to 2003 March in Cousins I with a 0.6 m telescope at Van Vleck Observatory. Twenty-eight periodic variables and 16 irregular variables have been identified. The variability study is most sensitive for stars with I < 14:3 mag; at that brightness level, we find that 24 of the 27 known pre-main-sequence (PMS) cluster members in the monitored field are variables, illustrating the value of photometric monitoring for identifying PMS cluster members. Among this brighter sample, 14 of the 16 known K or M-type weak-line T Tauri stars (WTTS) were found to be periodic variables, while all five of the known classical T Tauri stars (CTTS) were found to be irregular variables. In the full sample, which includes 150 stars with I as faint as 18, we find that 40% of the 63 WTTS are detected as variables, nearly all of them periodic, while 55% of the 20 CTTS are also detected as variable, with none of them periodic. Our study suggests that 80%-90% of all WTTS in young clusters will be detected as periodic variables given sufficiently precise and extended monitoring, whereas CTTS will reveal themselves primarily or solely as irregular variables. This has clear consequences for PMS rotational studies based on photometric periods, suggesting that any such sample may be biased against stars that are currently actively accreting (i.e., CTTS). We examine the stability of the periodic light curves from season to season. All periodic stars show modulations of their amplitude, mean brightness, and light-curve shape on timescales of less than 1 yr, presumably due to changes in spot configurations and/or physical characteristics. In no case, however, can we find definitive evidence of a change in period, indicating that differential rotation is probably much less in WTTS than it is in the Sun. While some stars show a hint of what could be cyclic behavior analogous to the sunspot cycle, no clear cycles could be found. It appears that most of the variation in light-curve shape is caused by redistribution of spots on the surface rather than by an increase or decrease in the areal spot coverage. While most of the variables are of K or M spectral class, we do confirm the existence of three, lowamplitude, periodic G stars. The rotation periods of these more massive stars are short compared to the bulk of the sample; it appears that mid-K to early M (i.e., $0.5 M ) represents a minimum in mean rotation rate for extremely young stars. Among the nonperiodic stars, we report the detection of two possible UX Orionis stars as well as a pre-main-sequence star, HMW 15, which apparently undergoes an eclipse with a duration exceeding 3 yr.

Results of an international campaign to photometrically monitor the unique pre-main sequence eclipsing object KH 15D are reported. An updated ephemeris for the eclipse is derived that incorporates a slightly revised period of 48.36 d. There is some evidence that the orbital period is actually twice that value, with two eclipses occurring per cycle. The extraordinary depth (∼3.5 mag) and duration (∼18 days) of the eclipse indicate that it is caused by circumstellar matter, presumably the inner portion of a disk. The eclipse has continued to lengthen with time and the central brightness reversals are not as extreme as they once were. V-R and V-I colors indicate that the system is slightly bluer near minimum light. Ingress and egress are remarkably well modeled by the passage of a knife-edge across a limb-darkened star. Possible models for the system are briefly discussed.Subject headings: stars: pre-main sequence -stars: circumstellar matterstars: individual (KH 15D)