Table of contents

A comparative study of spectra of 21 Type Ia supernovae (SNe Ia) obtained about 1 week before maximum light, and 8 spectra obtained 11 or more days before maximum, is presented. To a large extent, the premaximum spectra exhibit the defining characteristics of the four groups defined in Paper II (core normal, broad line, cool, and shallow silicon). Comparisons with SYNOW synthetic spectra show that all strong features and most weak ones can be accounted for in a plausible way. The issues of detached high‐velocity features, the possible ubiquity of carbon clumps, the maximum detectable ejecta velocities, and the possibility of blueshifted emission‐line peaks are discussed.

This paper presents measurements of the ¹²C/¹³C abundance ratio among red giants in the globular cluster Messier 71. The stars observed have absolute magnitudes of M_V<1.0. The measurements are based on spectroscopy of 2.3 μm ^12,13CO bands using the NIRSPEC instrument on the Keck II Telescope. The resulting isotope ratios agree well with values derived by Briley et al. from CN bands at 8005 Å. All of the red giants in the NIRSPEC sample with M_V<0 are found to have ¹²C/¹³C < 10; an anticorrelation is found to exist between this ratio and λ4215 CN band strength, as well as a correlation with oxygen abundance. There are three giants in our sample in the magnitude range + 1<M_V<0. These stars are found to have ¹²C/¹³C > 10, although the observational uncertainties are larger for such stars, due to both weaker CO bands and lower signal‐to‐noise ratios in the spectra. Thus, there may be some evidence that ¹²C/¹³C is decreasing with increasing stellar luminosity among the lower luminosity giants in our sample.

We studied differential single‐channel Strömgren uvby observations of four magnetic CP (mCP) stars, obtained with the Four College Automated Photoelectric Telescope (FCAPT), which now have two epochs of observations. To bring the recent photometry of BN Cam into agreement with earlier FCAPT photometry, we adjusted its period to 2.73501 days. As we found several older photometric studies whose data are consistent with this value, its period appears to have been constant for the past 35 years. We refined the period of EP Vir to 16.3071 days and reduced the errors in its ephemeris. When we compare the light curves from the observing seasons in which complete light curves were obtained, we find the small differences among them to be suggestive of variability. For FF Vir and HD 184905, the periods are 129.99 and 1.84535 days, respectively, as found by previous FCAPT data studies. Their light curves are similar to those of previous uvby and V measurements for the past 35 years.

We present new results from a radial velocity study of six bright OB stars with little or no prior measurements. One of these, HD 45314, may be a long‐period binary, but the velocity variations of this Be star may be related to changes in its circumstellar disk. Significant velocity variations were also found for HD 60848 (possibly related to nonradial pulsations) and HD 61827 (related to wind variations). The other three targets, HD 46150, HD 54879, and HD 206183, are constant‐velocity objects, but we note that HD 54879 has Hα emission that may originate from a binary companion. We illustrate the average red spectrum of each target.

We present a review of precursor observing programs for the SIM PlanetQuest Key Project devoted to detecting Jupiter‐mass planets around young stars. In order to ensure that the stars in the sample are free of various sources of astrometric noise that might impede the detection of planets, we have initiated programs to collect photometry, high‐contrast images, interferometric data, and radial velocities for stars in both the northern and southern hemispheres. We have completed a high‐contrast imaging survey of target stars in Taurus and the Pleiades and found no definitive common proper motion companions within 1^'' (140 AU) of the SIM targets. Our radial velocity surveys have shown that many of the target stars in Sco‐Cen are fast rotators, and a few stars in Taurus and the Pleiades may have substellar companions. Interferometric data of a few stars in Taurus show no signs of stellar or substellar companions with separations of 5–50 mas. The photometric survey suggests that approximately half of the stars initially selected for this program are variable to a degree (1 σ > 0.1 mag) that would degrade the astrometric accuracy achievable for that star. While the precursor programs are still a work in progress, we provide a comprehensive list of all targets and rank them according to their viability as a result of the observations taken to date. The observable that removes by far the most targets from the SIM young stellar object (YSO) program is photometric variability.

We describe how the James Webb Space Telescope (JWST) Near‐Infrared Spectrograph's (NIRSpec) detectors will be read out, and present a model of how noise scales with the number of multiple nondestructive reads sampling up the ramp. We believe that this noise model, which is validated using real and simulated test data, is applicable to most astronomical near‐infrared instruments. We describe some nonideal behaviors that have been observed in engineering‐grade NIRSpec detectors, and demonstrate that they are unlikely to affect NIRSpec sensitivity, operations, or calibration. These include a HAWAII‐2RG reset anomaly and random telegraph noise (RTN). Using real test data, we show that the reset anomaly is (1) very nearly noiseless and (2) can be easily calibrated out. Likewise, we show that large‐amplitude RTN affects only a small and fixed population of pixels. It can therefore be tracked using standard pixel operability maps.

Laser guide stars have increased the utility of adaptive optics systems by expanding the number of observable objects. The most common type of laser excites sodium in the mesosphere, and mesospheric sodium density is key to the performance of the laser. While a variety of observatories have conducted studies of the mesospheric sodium density, there are no published studies from Hawaii, which is home to some of the largest telescopes in the world. This paper presents mesospheric sodium densities measured by the University of Illinois lidar for 165 hr spanning 25 nights over 3 years. The mean sodium column density is 4.3 × 10⁹ ± 0.2 × 10⁹ cm⁻², with a seasonal peak in the winter, as found at many other sites. The variations in a given night can be as high as the seasonal variation. We predict the average photon returns for the 15 W Keck II laser and a proposed 50 W laser at the Advanced Electro‐Optical System 3.6 m telescope for the observed sodium abundances.

The site survey for the Advanced Technology Solar Telescope (ATST) of the National Solar Observatory was initiated in 2002 to find the best location for a 4 m aperture solar telescope. At the end of a 4 year survey, three sites (Big Bear Solar Observatory [BBSO] in California, Mees Solar Observatory [MSO] on Haleakala, Maui, Hawaii, and Observatorio Roque de los Muchachos, on La Palma, Spain) were identified as excellent sites for high‐resolution solar observations. MSO was ultimately chosen as the future ATST site. We present a subset of the ATST site survey data, focusing on the local seeing environment at BBSO. In particular, we are interested in the seeing characteristics at a mountain lake‐site observatory, its relation to the local environment and climate, and its implications for the 1.6 m New Solar Telescope (NST) currently being built at BBSO. We find a close correlation of very good seeing conditions with the prevailing wind direction and speed. The observatory building, located at the end of a 300 m causeway, is surrounded by the cool waters of Big Bear Lake, which effectively suppress the ground‐layer seeing. Very good seeing conditions from sunrise to sunset are a unique feature of BBSO, which makes it ideally suited for synoptic observations and sustained high‐resolution studies of solar activity and space weather.

A radio frequency interference (RFI) excision algorithm based on spectral kurtosis, a spectral variant of time‐domain kurtosis, is proposed and implemented in software. The algorithm works by providing a robust estimator for Gaussian noise that, when violated, indicates the presence of non‐Gaussian RFI. A theoretical formalism is used that unifies the well‐known time‐domain kurtosis estimator with past work related to spectral kurtosis, and leads naturally to a single expression encompassing both. The algorithm accumulates the first two powers of M power spectral density (PSD) estimates, obtained via Fourier transform, to form a spectral kurtosis (SK) estimator whose expected statistical variance is used to define an RFI detection threshold. The performance of the algorithm is theoretically evaluated for different time‐domain RFI characteristics and signal‐to‐noise ratios η. The theoretical performance of the algorithm for intermittent RFI (RFI present in R out of M PSD estimates) is evaluated and shown to depend greatly on the duty cycle, d = R/M. The algorithm is most effective for d = 1/(4 + η), but cannot distinguish RFI from Gaussian noise at any η when d = 0.5. The expected efficiency and robustness of the algorithm are tested using data from the newly designed FASR Subsystem Testbed radio interferometer operating at the Owens Valley Solar Array. The ability of the algorithm to discriminate RFI against the temporally and spectrally complex radio emission produced during solar radio bursts is demonstrated.