A lightweight vertical rosette for deployment in ice-covered waters

A lightweight modular rosette system has been developed that can be launched and recovered from aircraft in ice-covered waters through a 12 in. diameter hole in the ice. The small diameter is achieved by the modular design, in which a CTD module is attached to the end of a conducting cable and water bottle modules (four 4-L bottles per module) are positioned vertically above it. A novel tripping mechanism based on melting a link of monofilament line is used to close the water bottles at the desired depths. After launching the rosette, the cast proceeds like a normal rosette cast with the traces of temperature, salinity, oxygen and other desired sensors being displayed on a computer screen during the down and up casts and tripping the bottles electronically at the desired depths on the up cast. A Seabird 19+ CTD and Seabird 43 oxygen sensor are mounted in the CTD module and data acquisition and bottle tripping are controlled using a Seabird 33 deck unit and Seabird's SeaSave software run on a laptop computer. Deployment and recovery are done in a heated tent attached to the aircraft to prevent the water from freezing. After recovery the bottle modules are placed in coolers with bags of snow to stabilize the cooler temperature close to 0 °C, which is within ±1.8 °C of the in situ temperature, and the modules are transported back to a base camp for subsampling and sample processing. This system has been used to collect over 250 water samples in the ice-covered Lincoln Sea and the quality of the samples for dissolved gases and other constituents has been excellent.     

Photoionizing feedback in star cluster formation

We present the first ever hydrodynamic calculations of star cluster formation that incorporate the effect of feedback from ionizing radiation. In our simulations, the ionizing source forms in the cluster core at the intersection of several dense filaments of inflowing gas. We show that these filaments collimate ionized outflows and suggest such an environmental origin for at least some observed outflows in regions of massive star formation. Our simulations show both positive feedback (i.e. promotion of star formation in neutral gas compressed by expanding H  ii regions) and negative feedback (i.e. suppression of the accretion flow in to the central regions). We show that the volume filling factor of ionized gas is very different in our simulations from the result from the case where the central source interacted with an azimuthally smoothed gas density distribution. As expected, gas density is the key parameter in determining whether or not clusters are unbound by photoionizing radiation. Nevertheless, we find – on account of the acceleration of a small fraction of the gas to high velocities in the outflows – that the deposition in the gas of an energy that exceeds the binding energy of the cluster is not a sufficient criterion for unbinding the bulk of the cluster mass.     

Spectral evolution of microwaves and hard X-rays in the 1989 March 18 flare and its interpretation

We analyze the time variation of microwave spectra and hard X-ray spectra of 1989 March 18, which are obtained from the Solar Array at the Owens Valley Radio Observatory (OVRO) and the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM), respectively. From this observation, it is noted that the hard X-ray spectra gradually soften over 50–200 keV on-and-after the maximum phase while the microwaves at 1–15 GHz show neither a change in spectral shape nor as rapid a decay as hard X-rays. This leads to decoupling of hard X-rays from the microwaves in the decay phase away from their good correlation seen in the initial rise phase. To interpret this observation, we adopt a view that microwave-emitting particles and hard X-ray particles are physically separated in an inhomogeneous magnetic loop, but linked via interactions with the Whistler waves generated during flares. From this viewpoint, it is argued that the observed decoupling of microwaves from hard X-rays may be due to the different ability of each source region to maintain high energy electrons in response to the Whistler waves passing through the entire loop. To demonstrate this possibility, we solve a Fokker-Planck equation that describes evolution of electrons interacting with the Whistler waves, taking into account the variation of Fokker-Planck coefficients with physical quantities of the background medium. The numerical Fokker-Planck solutions are then used to calculate microwave spectra and hard X-ray spectra for agreement with observations. Our model results are as follows: in a stronger field region, the energy loss by electron escape due to scattering by the waves is greatly enhanced resulting in steep particle distributions that reproduce the observed hard X-ray spectra. In a region with weaker fields and lower density, this loss term is reduced allowing high energy electrons to survive longer so that microwaves can be emitted there in excess of hard X-rays during the decay phase of the flare. Our results based on spectral fitting of a flare event are discussed in comparison with previous studies of microwaves and hard X-rays based on either temporal or spatial information.     

Collisions and close encounters involving massive main-sequence stars

We study close encounters involving massive main-sequence stars and the evolution of the exotic products of these encounters as common-envelope systems or possible hypernova progenitors. We show that parabolic encounters between low- and high-mass stars and between two high-mass stars with small periastrons result in mergers on time-scales of a few tens of stellar free-fall times (a few tens of hours). We show that such mergers of unevolved low-mass stars with evolved high-mass stars result in little mass-loss  (∼0.01 M_⊙)  and can deliver sufficient fresh hydrogen to the core of the collision product to allow the collision product to burn for several million years. We find that grazing encounters enter a common-envelope phase which may expel the envelope of the merger product. The deposition of energy in the envelopes of our merger products causes them to swell by factors of ∼100. If these remnants exist in very densely populated environments  ( n ≳ 10⁷ pc⁻³)  , they will suffer further collisions which may drive off their envelopes, leaving behind hard binaries. We show that the products of collisions have cores rotating sufficiently rapidly to make them candidate hypernova/gamma-ray burst progenitors and that ∼0.1 per cent of massive stars may suffer collisions, sufficient for such events to contribute significantly to the observed rates of hypernovae and gamma-ray bursts.     

Analysis of the impulsive phase of a solar flare at submillimeter wavelengths

We present a report on the strong X5.3 solar flare which occurred on 25 August 2001, producing high-level γ-ray activity, nuclear lines and a dramatic long-duration white-light continuum. The bulk of millimeter radio fluxes reached a peak of ∼100 000 solar flux units at 89.4 GHz, and a few thousands of solar flux units were detected in the submillimeter range during the impulsive phase. In this paper we focus on and discuss (i) the implications inferred from high frequency radio observations during the impulsive phase; (ii) the dynamics of the low corona active region during the impulsive phase. In particular we found that 4–5 × 10³⁶ accelerated (>20 keV) electrons s⁻¹ radiating in a 1000–1100 G region, are needed to explain the millimeter to submillimeter-wave emissions. We present evidence that the magnetic field in the active region was very dynamic, and that strong non-thermal processes were triggered by the appearance of new, compact, low-lying (few thousand kilometers) loop systems, suggesting the acceleration site(s) were also located in the low solar atmosphere.     

Near-infrared spectra of laboratory H2O-CH4 ice mixtures

We present 1.25-19 μm infrared spectra of pure solid CH₄ and H₂O/CH₄=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH₄ in solid H₂O with those of pure CH₄. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH₄ in solid H₂O are estimated. Using the two largest (ν₃+ν₄) and (ν₁+ν₄) near-IR absorptions of CH₄ at 2.324 and 2.377 μm (4303 and 4207 cm⁻¹), respectively, as examples, we show that peaks of CH₄ in solid H₂O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH₄. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H₂O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH₄ absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies.     

Pressure-induced absorption by H2 in the atmosphere of Jupiter and Saturn

The S(1) line of the pressure-induced fundamental band of H₂ was identified and measured in the spectra of Saturn and Jupiter. This broad line at 4750 cm^?1 lies in a region free from telluric and planetary absorptions. It is about 99% absorbing in the core; the high-frequency wing extends to at least 5100 cm^?1. We compare the obseved line shape to the predictions of both a reflecting-layer model (RLM) and a homogeneous scattering model (HSM). The RLM provides a good fit to the Saturn line profile for temperatures near 150K; the derived base-level density is 0.52 (+0.26, ?0.17) amagat and the H₂ abundance is 25 (+10, ?9) km-amagat, assuming a scale height of 48 km. The Jupiter line profile is fit by both the RLM and HSM, but for widely differing temperatures, neither of which seems probable. The precise fitting of the observed S(1) line profile to computed models depends critically on the determination of the true continuum level; difficulties encountered in finding the continuum, especially for Jupiter, are discussed. Derived RLM densities and abundances for both planets are substantially lower than those derived from RLM analyses of the H₂ quadrupole lines, the 3ν₃ band of CH₄, and from other sources.