P. J. Puxley

The He I 2.06 microns/Br-gamma ratio in starburst galaxies - An objective constraint on the upper mass limit to the initial mass function

The use of the He I 2.06 microns/Br-gamma ratio as a constraint on the massive stellar population in star-forming galaxies is developed. A theoretical relationship between the He I 2.06 microns/Br-gamma ratio and the effective temperature of the exciting star in H II regions is derived. The effects of collisional excitation and dust within the nebula on the ratio are also considered. It is shown that the He I 2.06 microns/Br-gamma ratio is a steep function of the effective temperature, a property which can be used to determine the upper mass limit of the initial mass function (IMF) in galaxies. This technique is reliable for upper mass limits less than about 40 solar masses. New near-infrared spectra of starburst galaxies are presented. The He I 2.06 microns/Br-gamma ratios observed imply a range of upper mass limits from 27 to over 40 solar masses. There is also evidence that the upper mass limit is spatially dependent within a given galaxy. These results suggest that the upper mass limit is not a uniquely defined parameter of the IMF and probably varies with local physical conditions.

Near-infrared spectroscopy of the ultracompact H ii region1 G45.12 + 0.13

We present complete, low resolution IJHK spectroscopy of the ultracompact HII region, G45.12+0.13. From the observed HI line strengths, we derive a near infrared extinction law that is slightly steeper than the average. After correction with this extinction law, we find good agreement between the observed line ratios of HeI, Fe+, Fe++, S+ and S++ and the available atomic data. Our data show that the density within the core of G45.12+0.13 must be at least 10^4/cm^3. This is consistent with the known radio structure of the HII region and in considerable disagreement with previous work using mid and far infrared lines. There must also be considerable opacity in the HeI 2 3P-2 3S transition, and we show how the observed strengths of the other HeI lines are consistent with this. From modelling the photoionisation structure, we find good agreement with most of the observed data if the hottest star present has Teff 38000K. Discrepancies still exist between some of the observed and model line ratios, but the most obvious tend to be the mid-IR observations.

The Near-Infrared Extinction Law in Regions of High AV

We present a spectroscopic study of the shape of the dust-extinction law between 1.0 and 2.2µm towards a set of nine ultracompact H ii regions with AV � 15 mag. We find some evidence that the reddening curve may tend to flatten at higher extinctions, but just over half of the sample has extinction consistent with or close to the average for the interstellar medium. There is no evidence of extinction curves significantly steeper than the standard law, even where water ice is present. Comparing the results to the predictions of a simple extinction model, we suggest that a standard extinction law implies a robust upper limit to the grain-size distribution at around 0.1 0.3µm. Flatter curves are most likely due to changes in this upper limit, although the effects of flattening due to unresolved clumpy extinction cannot be ruled out.

Fluorescent Molecular Hydrogen in the Extragalactic Giant H II Region NGC 5461

We present a sensitive 2 μm spectrum of the extragalactic giant H II complex NGC 5461. The spectrum contains detections of vibrationally excited molecular hydrogen transitions from energy levels to 19000 K. Comparison of the line emission with models of excitation in photon-dominated regions demonstrates unambiguously that the gas is excited by the UV light from the newly formed stars. We derive an abundance ratio of ortho-to-para states in the excited gas of 1.15 ± 0.11, which is among the lowest ever recorded.

Distributed user support and the Gemini Observatory help desk

The Gemini Observatory HelpDesk was activated early in 2000 to aid in the rapid and accurate resolution of queries concerning the Gemini telescopes and their capabilities. This system co- ordinates user support amongst staff within the Observatory and at National Offices in each partner country. The HelpDesk is based on a commercial product from Remedy Corporation that logs, tracks, forwards and escalates queries and self- generates a knowledgebase of previously asked questions. Timestamping of these events in the life cycle of a request and analysis of associated information provides valuable feedback on the static web content and performance of user support.

Observing with a 21st century ground-based telescope--or how to do unique science with the Gemini Telescopes

The new generation of ground-based telescopes offer unique observational opportunities for astronomy. By adopting the paradigm that observations must be matched to conditions it will be possible to use an 8 m telescope to couple the spatial resolution of the Hubble space telescope with a least 10 times the collecting area. To effectively exploit these characteristics will require a considerable degree of pre- planning and prediction of environmental and atmospheric conditions combined with the ability to dynamically schedule observations. In this paper we describe the approach being taken by the Gemini telescopes to implement this new observing mode which is essential to realize Geminis ambitious science requirements.

Gemini Observatory science operations plan

We review the Gemini Observatory science operations plan including the proposal submission, allocation and observation planning processes; the telescope operation model; and the scientific staffing plans and user support. Use of the telescope is shown via a sub-stellar companion search program to illustrate the planning tools and level of integration required between the observatory control, telescope control and data handling software systems.

Support capability requirements of 8-m-telescope science

Science workshops were held throughout the Gemini partnership during the second half of 1997 with the aims of identifying and quantifying the supporting capabilities required to enhance the utility and efficiency of the Gemini 8m telescopes. These workshops, held separately in the US, UK, Canada and South America, ensured representation programs were considered in detail sufficient to understand the requirements for their execution on Gemini as well as for any preparatory observations. The desire for wide-field optical and near-IR imaging was frequently identified with an average of one-half to one night of these survey observations per night of Gemini follow-up. Two other common themes were high angular resolution imaging and rapid response to target-of-opportunity events.

Is C/O Enhanced in NGC 253?

We have mapped the nuclear region of the starburst galaxy NGC 253 in the3P1 →3P0 line of neutral carbon using the JCMT. Carbon is widespread across the nuclear region with a similiar distribution to CO as expected. Previous studies of Galactic star-forming regions showed that carbon emission is enhanced in photon-dominated regions (where UV photons impinge upon molecular clouds). Previous observations of other PDR tracers such as ionized carbon and FIR continuum constrain the physical conditions in the PDR gas of NGC 253. The carbon we have observed is far brighter than predicted by theoretical models of PDRs with solar elemental values. This indicates that carbon emission is not a reliable diagnostic of the physical conditions in the nuclear region of a galaxy undergoing a burst of star formation.