Randall D. Campbell

A low density of 0.8 g cm-3 for the Trojan binary asteroid 617 Patroclus

The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 stable Lagrange points of the Jupiter–Sun system (leading and following Jupiter by 60°). The asteroid 617 Patroclus is the only known binary Trojan. The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the systems centre of mass, describing a roughly circular orbit. Using this orbital information, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8 - 0.1 + 0.2 g cm-3. The components of 617 Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the Solar System.

Detection of Galactic Center source G2 at 3.8 μm during periapse passage

We report new observations of the Galactic Center source G2 from the W. M. Keck Observatory. G2 is a dusty red object associated with gas that shows tidal interactions as it nears closest approach with the Galaxy’s central black hole. Our observations, conducted as G2 passed through periapse, were designed to test the proposal that G2 is a 3 earth mass gas cloud. Such a cloud should be tidally disrupted during periapse passage. The data were obtained using the Keck II laser guide star adaptive optics system (LGSAO) and the facility near-infrared camera (NIRC2) through the K’ [2.1 µm] and L’ [3.8 µm] broadband filters. Several results emerge from these observations: 1) G2 has survived its closest approach to the black hole as a compact, unresolved source at L’; 2) G2’s L’ brightness measurements are consistent with those over the last decade; 3) G2’s motion continues to be consistent with a Keplerian model. These results rule out G2 as a pure gas cloud and imply that G2 has a central star. This star has a luminosity of �30 L⊙ and is surrounded by a large (�2.6 AU) optically thick dust shell. The differences between the L’ and Br- observations can be understood with a model in which L’ and Br- emission arises primarily from internal and external heating, respectively. We suggest that G2 is a binary star merger product and will ultimately appear similar to the B-stars that are tightly clustered around the black hole (the so-called S-star cluster). Subject headings: Galaxy: center — Techniques: photometric — Techniques: high angular resolution — Galaxy: nucleus — Infrared: stars — Black hole physics

AO operations at the W. M. Keck Observatory

Laser Guide Star Adaptive Optics (LGS AO) has been offered to Keck II visiting astronomers since November 2004. From the few nights of shared-risk science offered at that time, the LGS AO operation effort has grown to supporting over fifty nights of LGS AO per semester. In this paper we describe the new technology required to support LGS AO, give an overview of the operational model, report observing efficiency and discuss the support load required to operate LGS AO. We conclude the paper by sharing lessons learned and the challenges yet to be faced.

LGS AO at W. M. Keck Observatory: routine operations and remaining challenges

The Laser Guide Star Adaptive Optics (LGS AO) at the W.M. Keck Observatory is the first system of its kind being used to conduct routine science on a ten-meter telescope. In 2005, more than fifty nights of LGSAO science and engineering were carried out using the NIRC2 and OSIRIS science instruments. In this paper, we report on the typical performance and operations of its LGS AO-specific sub-systems (laser, tip-tilt sensor, low-bandwidth wavefront sensor) as well as the overall scientific performance and observing efficiency. We conclude the paper by describing our main performance limitations and present possible developments to overcome them.

Modeling anisoplanatism in the Keck II laser guide star AO system

Anisoplanatism is a primary source of photometric and astrometric error in single-conjugate adaptive optics. We present initial results of a project to model the off-axis optical transfer function in the adaptive optics system at the Keck II telescope. The model currently accounts for the effects of atmospheric anisoplanatism in natural guide star observations. The model for the atmospheric contribution to the anisoplanatic transfer function uses contemporaneous MASS/ DIMM measurements. Here we present the results of a validation campaign using observations of naturally guided visual binary stars under varying conditions, parameterized by the r0 and θ0 parameters of the C2n atmospheric turbulence profile. We are working to construct a model of the instrumental field-dependent aberrations in the NIRC2 camera using an artificial source in the Nasmyth focal plane. We also discuss our plans to extend the work to laser guide star operation.

Laser Operations at the 8-10m class telescopes Gemini, Keck and the VLT: lessons learned, old and new challenges

Laser Guide Star (LGS) assisted Adaptive Optics routine operations have commenced at three of the major astronomical observatories, in 2004 (Keck) and 2006 (VLT and Gemini) respectively. Subaru is also on the verge of putting its LGS facility into operations. In this paper we concentrate on the operational aspect of the laser facilities: we discuss common problems such as weather constraints, beam collisions, aircraft avoidance and optimal telescope scheduling. We highlight important differences between the observatories, especially in view of the valuable lessons learnt. While it is true that the three observatories have made quick progress and achieved important scientific results during the first years of operations, there is much room left for improvement in terms of the efficiency that can be obtained on sky. We compare and contrast the more recently implemented LGS systems of VLT and Gemini operated in service and queue modes to the more mature LGS operation at Keck that employs classical PI scheduled observing.

Characterization of the Si: As Blocked Impurity Band (BIB) Detector in Keck’s Long Wavelength Spectrometer (LWS)

Kecks Long Wavelength Spectrometer (LWS), is the facility instrument used for imaging and spectroscopy in the wavelength range of 3–28 μm at the Keck Observatory. LWS uses an 128 × 128 Si:As blocked impurity band (BIB) array manufactured by the Boeing Corporation. This paper discusses the method used for optimizing the detectors operating parameters at a temperature of 8.5 K and bias voltage of 1.2 V. A process for characterizing detective quantum efficiency of BIB detectors is also presented.

Modeling instrumental field-dependent aberrations in the NIRC2 instrument on the Keck II telescope

We present a model of field-dependent aberrations arising in the NIRC2 instrument on the W. M. Keck II telescope. We use high signal-to-noise phase diversity data employing a source in the Nasmyth focal plane to construct a model of the optical path difference as a function of field position and wavelength. With a differential wavefront error of up to 190 nm, this effect is one of the main sources of astrometric and photometric measurement uncertainties. Our tests of temporal stability show sufficient reliability for our measurements over a 20-month period at the field extrema. Additionally, while chromaticity exists, applying a correction for field-dependent aberrations provides overall improvement compared to the existing aberrations present across the field of view.

The Effects of Charge Persistence in Aladdin III InSb Detectors on Scientific Observations

Charge persistence is defined as the signal that remains on a detector array after the illumination source has been removed. It is characteristic of many types of photo detectors, but is particularly prominent in near-infrared arrays that utilize Indium Antimonide (InSb) as their photosensitive material. Persistence can be a significant issue for scientific observation due to the potential reduction of data quality. We have studied the properties of persistence in an Aladdin III array from Raytheon which is installed in Keck Observatorys adaptive optics camera, NIRC2. In this paper we discuss the types of observations that are at risk from persistence effects. We present data on the characteristics, which include the following results: 1) the level of persistence is more strongly influenced by incident flux than that of fluence (flux × integration time), 2) the persistence is wavelength independent, 3) the persistence level is not reduced by continuous reset-reads of the array or power law and is proportional to 1/t.

A potential progenitor for the Type Ic supernova 2017ein

We report the first detection of a credible progenitor system for a Type Ic supernova (SN Ic), SN 2017ein. We present spectra and photometry of the SN, finding it to be similar to carbon-rich, low-luminosity SNe Ic. Using a post-explosion Keck adaptive optics image, we precisely determine the position of SN 2017ein in pre-explosion \hst\ images, finding a single source coincident with the SN position. This source is marginally extended, and is consistent with being a stellar cluster. However, under the assumption that the emission of this source is dominated by a single point source, we perform point-spread function photometry, and correcting for line-of-sight reddening, we find it to have