Adrian Webster

Profiles of the λ6196 and λ6379 Diffuse Interstellar Bands

We have looked for rotational structure in the sharp λ6196 and λ6379 diffuse interstellar bands (DIBs) at a resolution ~120,000 in seven stars where the interstellar λ7699 K I line is unresolved. The λ6196 DIB is bell-shaped with a flat core and differs slightly in width from star to star. It is accompanied by a weak DIB at λ6194.7, with which it does not maintain a constant depth ratio. The λ6379 DIB is asymmetric with a sharp double core, but the profile hardly varies between the stars apart from being undetectable for HD 37061. Weak features connect it to the weaker λ6376 DIB, with which it varies in unison. Simple rotational models do not fit the observed profiles of λ6196 and λ6376 at all well because of more prominent branch structure in the models. We achieve an acceptable fit by arbitrarily convolving the modeled profiles with Gaussians (0.2 to 0.3 cm-1). The Gaussians correspond to an unexpected or anomalous broadening process that cannot be explained by the interstellar velocity distribution or the instrumental point-spread function. The fits give upper limits to the ratio of the rotational temperature to the moment of inertia of the molecular carriers. If the former lies in the range 10-100 K, then the molecules must be large, with moments of inertia comparable with that of the fullerene C60. We present evidence that the anomalous broadening of the rotational profiles is intramolecular in origin, but it is not easily explained by broadening processes previously invoked in connection with the DIBs. We suggest that some other process such as a zero-point vibrational isotope shift may be involved that could be characteristic of many of the narrow bands.

Birch reduction of C60—a new appraisal

Contrary to a previous report that Birch reduction of C60 affords C60H36 as the principal product, laser desorption-laser photoionisation time-of-flight (L2TOF), laser desorption Fourier transform ion cyclotron resonance (FTICR), and liquid secondary ion mass spectrometry (LSIMS) show collectively that a mixture of polyhydrofullerenes, containing C60H18 through to C60H36 with a skewed distribution centred on C60H32 is formed, the discrepancy in results arising from the thermal lability of this mixture of polyhydrofullerenes when subjected to the elevated temperatures (>250 °C) required for mass spectroscopic studies using direct-insertion heated probes.

Precision radial velocity spectrograph

We present a conceptual design for a Precision Radial Velocity Spectrograph (PRVS) for the Gemini telescope. PRVS is a fibre fed high resolving power (R~70,000 at 2.5 pixel sampling) cryogenic echelle spectrograph operating in the near infrared (0.95 - 1.8 microns) and is designed to provide 1 m/s radial velocity measurements. We identify the various error sources to overcome in order to the required stability. We have constructed models simulating likely candidates and demonstrated the ability to recover exoplanetary RV signals in the infrared. PRVS should achieve a total RV error of around 1 m/s on a typical M6V star. We use these results as an input to a simulated 5-year survey of nearby M stars. Based on a scaling of optical results, such a survey has the sensitivity to detect several terrestrial mass planets in the habitable zone around nearby stars. PRVS will thus test theoretical planet formation models, which predict an abundance of terrestrial-mass planets around low-mass stars.We have conducted limited experiments with a brass-board instrument on the Sun in the infrared to explore real-world issues achieving better than 10 m/s precision in single 10 s exposures and better than 5 m/s when integrated across a minute of observing.

Precision Radial Velocities in the Infrared

Over 250 extra-solar planets have been discovered to date using a variety of techniques. The majority have been discovered at optical wavelengths from the Doppler shift of F, G and K stars induced by orbiting planets. We have constructed models simulating likely planets around M dwarfs and demonstrated the ability to recover their radial velocity signals in the infrared. We have conducted experiments in the infrared with a brass-board instrument to explore real-world issues. We are thus confident that a stabilised radial velocity spectrometer with a single-shot 1 and 1.7 microns coverage at a resolution of around 70 k can achieve an instrumental radial velocity error of 0.5 m/s. This enables the efficient measurement of radial velocities for M, L and T spectral classes. We have modelled the radial velocity information in low-mass star spectra and checked our ability to recover this signal in the face of the telluric contamination in the infrared. Including instrumental error, telluric contamination and photon noise we predict a total radial velocity error of less than 2 m/s on a typical M6V star at 10 pc. We use these results as an input to a simulated 5-year survey of nearby M stars envisaged for Gemini Observatory. Based on a conservative scaling of optical results, such a survey has sensitivity to detect several terrestrial mass planets in the habitable zone around nearby stars. It can test theoretical planet formation models, which predict an over-abundance of terrestrial-mass planets around low-mass stars. Improvements in the efficiency and sampling of searches at optical wavelengths promise long-term precisions of 0.5 m/s and 5 MEarth detections around solartype stars. While this may be the limit for CCD-based surveys of solar type stars until larger telescopes become available it is nonetheless feasible to survey lower mass primaries to achieve a corresponding smaller mass limit. Thus the lowest mass Doppler signals have been found around M dwarfs (e.g., GJ581b 5 MEarth sin i [1]) and detections down to a few MEarth detections should be feasible around mid-type M dwarfs. This is the Precision Radial Velocity Spectrometer (PRVS) approach: to search around lower-mass primary stars since the radial velocity signal will be larger for lighter primary stars (http://www.roe.ac.uk/ukatc/projects/prvs). Based on the spectral information in real and synthetic spectra of M dwarfs, PRVS is designed to measure the peak of their energy distribution. We find that for low rotation M dwarfs a resolution of around 70 000 is optimum. This leads to us