Table of contents

We review the observed properties of extremely hot, hydrogen‐deficient post–asymptotic giant branch (AGB) stars of spectral type [WC] and PG1159. Their H deficiency is probably caused by a (very) late helium‐shell flash or an AGB final thermal pulse, laying bare interior stellar regions that are usually kept hidden below the hydrogen envelope. Thus, the photospheric elemental abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We summarize the state of the art of stellar evolution models that simulate AGB evolution and the occurrence of a late He‐shell flash. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non‐LTE model atmospheres. Good qualitative and quantitative agreement is found. Future work can contribute to an even more complete picture of the nuclear processes in AGB stars.

To use Type Ia supernovae as standard candles for cosmology, we need accurate broadband magnitudes. In practice the observed magnitude may differ from the ideal magnitude‐redshift relationship either through intrinsic inhomogeneities in the Type Ia supernova population, or through observational error. Here we investigate how we can choose filter bandpasses to reduce the error caused by both these effects. We find that bandpasses with large integral fluxes and sloping wings are best able to minimize several sources of observational error, and are also least sensitive to intrinsic differences in Type Ia supernovae. The most important feature of a complete filter set for Type Ia supernova cosmology is that each bandpass be a redshifted copy of the first. We design practical sets of redshifted bandpasses that are matched to typical high‐resistivity CCD and HgCdTe infrared detector sensitivities. These are designed to minimize systematic error in well‐observed supernovae; final designs for specific missions should also consider signal‐to‐noise ratio requirements and observing strategy. In addition, we calculate how accurately filters need to be calibrated in order to achieve the required photometric accuracy of future supernova cosmology experiments, such as the Supernova /Acceleration Probe (SNAP), which is one possible realization of the Joint Dark Energy Mission (JDEM). We consider the effect of possible periodic miscalibrations that may arise from the construction of an interference filter.

We report progress in the calibration of a method to determine cool dwarf star metallicities using molecular band strength indices. The molecular band index to metallicity relation can be calibrated using chemical abundances calculated from atomic‐line equivalent width measurements in high‐resolution spectra. Building on previous work, we have measured Fe and Ti abundances in 32 additional M and K dwarf stars to extend the range of temperature and metallicity covered. A test of our analysis method using warm star–cool star binaries shows we can calculate reliable abundances for stars warmer than 3500 K. We have used abundance measurements for warmer binary or cluster companions to estimate abundances in six additional cool dwarfs. Adding stars measured in our previous work and others from the literature provides 76 stars with Fe abundance and CaH2 and TiO5 index measurements. The CaH2 molecular index is directly correlated with temperature. TiO5 depends on temperature and metallicity. Metallicity can be estimated to within ±0.3 dex within the bounds of our calibration, which extends from roughly [Fe/H] = +0.05 to −1.0, with a limited extension to −1.5.

We present optical spectra of a flare on Barnard's star. Several photospheric and chromospheric species were enhanced by the flare heating. An analysis of the Balmer lines shows that their shapes are best explained by Stark broadening rather than chromospheric mass motions. We estimate the temperature of the flaring region in the lower atmosphere to be ≥8000 K and the electron density to be ∼10¹⁴ cm⁻³, similar to values observed in other dM flares. Because Barnard's star is considered to be one of our oldest neighbors, a flare of this magnitude is probably quite rare.

The cataclysmic variable ASAS J002511+1217.2 was discovered in outburst by the All‐Sky Automated Survey (ASAS) in 2004 September, and intensively monitored by AAVSO observers through the following 2 months. Both photometry and spectroscopy indicate that this is a very short period system. Clearly defined superhumps with a period of 0.05687 ± 0.00001 (1 σ) days (81.9 minutes) are present during the superoutburst, 5 to 18 days following the ASAS detection. We observe a change in superhump profile similar to the transition to "late superhumps" observed in other short‐period systems; the superhump period appears to increase slightly for a time before returning to the original value, with the resulting superhump phase offset by approximately half a period. We detect variations with a period of 0.05666 ± 0.00003 (1 σ) days (81.6 minutes) during the 4 day quiescent phase between the end of the main outburst and the single echo outburst. Weak variations having the original superhump period reappear during the echo and its rapid decline. Time‐resolved spectroscopy conducted nearly 30 days after detection and well into the decline yields an orbital period measurement of 82 ± 5 minutes. Both narrow and broad components are present in the emission‐line spectra, indicating the presence of multiple emission regions. The weight of the observational evidence suggests that ASAS J002511+1217.2 is a WZ Sge–type dwarf nova, and we discuss how this system fits into the WZ classification scheme.

Highly accurate astrometric positions of 14 of Saturn's satellites have been obtained from 444 Hubble Space Telescope images taken with the Wide Field Planetary Camera 2 (WFPC2) between 1996 and 2005. In all, 1036 satellite positions were measured in Planetary Camera (PC) frames, with a typical uncertainty of σ_PC = 0014 (80 km at Saturn), and 1403 positions from Wide Field (WF) frames, with σ_WF = 0020 (120 km at Saturn). A key part of the reduction involved the application of an improved WFPC2 distortion‐correction scheme (Anderson & King) and precise determination of the relative positions of the PC and WF chips, which varied substantially over the full course of the observation period. The time span covered by the observations is more than twice the nominal duration of the Cassini mission and thus provides an important baseline of measurements that is particularly important for studying time‐variable phenomena such as the orbital exchange of Janus and Epimetheus and the chaotic interactions of Prometheus and Pandora. These results have been incorporated into ephemerides that are being used for planning and analysis of Cassini satellite and ring observations.

An increasing amount of the literature reports the detection of magnetic fields in asymptotic giant branch (AGB) stars and in central stars of planetary nebulae (PNe). These detections lead to claims that the magnetic fields are the main agent shaping the PNe. In this paper, I examine the energy and angular momentum carried by magnetic fields expelled from AGB stars, as well as other physical phenomena that accompany the presence of large‐scale fields, such as those claimed in the literature. I show that a single star cannot supply the energy and angular momentum if the magnetic fields have the large coherent structure required to shape the circumstellar wind. Therefore, the structure of nonspherical planetary nebulae cannot be attributed to dynamically important large‐scale magnetic fields. I conclude that the observed magnetic fields around evolved stars can be understood with respect to locally enhanced magnetic loops, which can have a secondary role in the shaping of the PN. The primary role, I argue, rests with the presence of a companion.

The Advanced Camera for Surveys on board the Hubble Space Telescope is equipped with one grism and three prisms for low‐resolution, slitless spectroscopy in the range 1150–10500 Å. The G800L grism provides optical spectroscopy between 5500 Å and >1 μm, with a mean dispersion of 39 and 24 Å pixel⁻¹ (in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on‐board calibration lamps for wavelength and narrowband flat‐fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention given to the treatment of the field dependence of the grism flat field, wavelength solution, and sensitivity in both channels.

The 2 m Liverpool Telescope (LT), owned by Liverpool John Moores University, is located in La Palma (Canary Islands) and operates in fully robotic mode. In 2005, the LT began conducting an automatic gamma‐ray burst (GRB) follow‐up program. On receiving an automatic GRB alert from a gamma‐ray observatory (Swift, INTEGRAL, HETE‐2, or IPN), the LT initiates a special override mode that conducts follow‐up observations within 2–3 minutes of the GRB onset. This follow‐up procedure begins with an initial sequence of short (10 s) exposures acquired through an r^' band filter. These images are reduced, analyzed, and interpreted automatically using pipeline software developed by our team, called LT‐TRAP (Liverpool Telescope Transient Rapid Analysis Pipeline); the automatic detection and successful identification of an unknown and potentially fading optical transient triggers a subsequent multicolor imaging sequence. In the case of a candidate brighter than r^' = 15, either a polarimetric (from 2006) or a spectroscopic observation (from 2007) will be triggered on the LT. If no candidate is identified, the telescope continues to obtain z^', r^', and i^' band imaging with increasingly longer exposure times. Here we present a detailed description of the LT‐TRAP and briefly discuss the illustrative case of the afterglow of GRB 050502a, whose automatic identification by the LT just 3 minutes after the GRB led to the acquisition of the first early‐time (<1 hr) multicolor light curve of a GRB afterglow.

The Keck Observatory began science observations with a laser guide star adaptive optics system, the first such system on an 8–10 m class telescope, in late 2004. This new capability greatly extends the scientific potential of the Keck II Telescope, allowing near–diffraction‐limited observations in the near‐infrared using natural guide stars as faint as 19th magnitude. This paper describes the conceptual approach and technical implementation followed for this system, including lessons learned, and provides an overview of the early science capabilities.

The Keck II Telescope is the first 8–10 m class telescope equipped with a laser guide star adaptive optics (LGS AO) system. Under normal seeing conditions, the LGS AO system produces K‐band Strehl ratios between 30% and 40% using bright tip‐tilt guide stars, and it works well with tip‐tilt guide stars as faint as m_R = 18, with partial correction for stars up to a magnitude fainter. This paper presents the algorithms implemented in the LGS AO system, as well as experimental performance results. A detailed error budget shows excellent agreement between the measured and expected image quality for both bright and faint guide stars.

A real‐valued genetic algorithm with random rank‐based selection is shown to successfully estimate the multiple phases of a segmented optical system modeled on the seven‐mirror Systematic Image‐Based Optical Alignment test bed located at NASA's Marshall Space Flight Center. Comparisons are made between this and more traditional phase‐retrieval methods. No significant increase in computational speed is observed using the genetic algorithm technique.

An all‐sky survey in two mid‐infrared bands covering wavelengths from 6 to 12 and 14 to 26 μm, with a spatial resolution of ∼94–10^'', will be performed with the Infrared Camera (IRC) on board the ASTRO‐F infrared astronomical satellite. The expected detection limit for point sources is 80–130 mJy (5 σ). The all‐sky survey will provide data with a detection limit and a spatial resolution an order of magnitude deeper and higher, respectively, than those of the Infrared Astronomical Satellite survey. The IRC is optimally designed for deep imaging in staring observations. It employs 256 × 256 Si:As IBC infrared focal plane arrays for the two mid‐infrared channels. In order to make observations with the IRC during the scanning observations for the all‐sky survey, a new method of operation for the arrays has been developed—"scan mode" operation. In the scan mode, only 256 pixels in a single row aligned in the cross‐scan direction on the array are used as the scan detector, and they are sampled every 44 ms. Special care has been taken to stabilize the temperature of the array in scan mode, which enables the user to achieve a low readout noise, comparable to that in the imaging mode (20–30 e⁻). The accuracy of the position determination and the flux measurement for point sources is examined both in computer simulations and laboratory tests with the flight model camera and moving artificial point sources. In this paper we present the scan mode operation of the array, the results of the computer simulation and the laboratory performance test, and the expected performance of the IRC all‐sky survey observations.

We report site‐testing results obtained in the nighttime during the polar autumn and winter at Dome C. These results were collected during the first Concordia winterover by A. Agabi. They are based on seeing and isoplanatic angle monitoring, as well as in situ balloon measurements of the refractive index structure constant profiles C²_n(h). Atmosphere is divided into two regions: (1) a 36 m high surface layer responsible for 87% of the turbulence, and (2) a very stable free atmosphere above, with a median seeing of 036 ± 019 at an elevation of h = 30 m. The median seeing measured with a differential image motion monitor placed on top of an 8.5 m high tower is 13 ± 08.