Deuterium-enriched water ties planet-forming disks to comets and protostars.

Water is a fundamental molecule in the star and planet formation process, essential for catalysing the growth of solid material and the formation of planetesimals within disks1,2. However, the water snowline and the HDO:H2O ratio within proto-planetary disks have not been well characterized because water only sublimates at roughly 160 K (ref. 3), meaning that most water is frozen out onto dust grains and that the water snowline radii are less than 10 AU (astronomical units)4,5. The sun-like protostar V883 Ori (M* = 1.3 M⊙)6 is undergoing an accretion burst7, increasing its luminosity to roughly 200 L⊙ (ref. 8), and previous observations suggested that its water snowline is 40-120 AU in radius6,9,10. Here we report the direct detection of gas phase water (HDO and [Formula: see text]) from the disk of V883 Ori. We measure a midplane water snowline radius of approximately 80 AU, comparable to the scale of the Kuiper Belt, and detect water out to a radius of roughly 160 AU. We then measure the HDO:H2O ratio of the disk to be (2.26 ± 0.63) × 10-3. This ratio is comparable to those of protostellar envelopes and comets, and exceeds that of Earth's oceans by 3.1σ. We conclude that disks directly inherit water from the star-forming cloud and this water becomes incorporated into large icy bodies, such as comets, without substantial chemical alteration.

EXPLORING THE ORIGINS OF DEUTERIUM ENRICHMENTS IN SOLAR NEBULAR ORGANICS.

Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues about their original formation environment. The organic materials in primitive solar system bodies generally have higher D/H ratios and show greater D/H variation when compared to D/H in solar system water. We propose this difference arises at least in part due to (1) the availability of additional chemical fractionation pathways for organics beyond that for water, and (2) the higher volatility of key carbon reservoirs compared to oxygen. We test this hypothesis using detailed disk models, including a sophisticated, new disk ionization treatment with a low cosmic-ray ionization rate, and find that disk chemistry leads to higher deuterium enrichment in organics compared to water, helped especially by fractionation via the precursors CH 2 D + / CH 3 + . We also find that the D/H ratio in individual species varies significantly depending on their particular formation pathways. For example, from ~20-40 au, CH4 can reach D/H ~ 2 × 10-3, while D/H in CH3OH remains locally unaltered. Finally, while the global organic D/H in our models can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir, our models are unable to reproduce the most deuterium-enriched organic materials in the solar system, and thus our model requires some inheritance from the cold interstellar medium from which the Sun formed.

Exploring the origins of carbon in terrestrial worlds.

Given the central role of carbon in the chemistry of life, it is a fundamental question as to how carbon is supplied to the Earth, in what form and when. We provide an accounting of carbon found in solar system bodies, and in particular a comparison between the organic content of meteorites and that in identified organics in the dense interstellar medium (ISM). Based on this accounting, identified organics created by the chemistry of star formation could contain at most -15% of the organic carbon content in primitive meteorites and significantly less for cometary organics, which represent the putative contributors to starting materials for the Earth. In the ISM -30% of the elemental carbon exists as CO, either in gaseous form or in ices, with a typical abundance of -10(-4) (relative to H2). Recent observations of the TW Hya disk find that the gas phase abundance of CO is reduced by an order of magnitude compared to this value. We explore an explanation for this observation whereby the volatile CO is destroyed via gas phase processes, providing an additional source of carbon for organic material to be incorporated into planetesimals and cometesimals. This chemical processing mechanism requires warm grains (> 20 K), partially ionized gas, and sufficiently small (a(grain)) < 10 microm) grains, ie. a larger total grain surface area, such that freeze-out is efficient. Under these conditions, static (non-turbulent) chemical models predict that a large fraction of the carbon nominally sequestered in CO can be the source of carbon for a wide variety of organics that are present as ice coatings on the surfaces of warm pre-planetesimal dust grains.

The ancient heritage of water ice in the solar system.

Identifying the source of Earth's water is central to understanding the origins of life-fostering environments and to assessing the prevalence of such environments in space. Water throughout the solar system exhibits deuterium-to-hydrogen enrichments, a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent molecular cloud or (ii) the solar nebula protoplanetary disk. Using a comprehensive treatment of disk ionization, we find that ion-driven deuterium pathways are inefficient, which curtails the disk's deuterated water formation and its viability as the sole source for the solar system's water. This finding implies that, if the solar system's formation was typical, abundant interstellar ices are available to all nascent planetary systems.

An old disk still capable of forming a planetary system.

From the masses of the planets orbiting the Sun, and the abundance of elements relative to hydrogen, it is estimated that when the Solar System formed, the circumstellar disk must have had a minimum mass of around 0.01 solar masses within about 100 astronomical units of the star. (One astronomical unit is the Earth-Sun distance.) The main constituent of the disk, gaseous molecular hydrogen, does not efficiently emit radiation from the disk mass reservoir, and so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally indicates properties of the disk surface, and the conversion from dust emission to gas mass requires knowledge of the grain properties and the gas-to-dust mass ratio, which probably differ from their interstellar values. As a result, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3-10 million years) star TW Hydrae, for which the range is 0.0005-0.06 solar masses. Here we report the detection of the fundamental rotational transition of hydrogen deuteride from the direction of TW Hydrae. Hydrogen deuteride is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The detection of hydrogen deuteride, combined with existing observations and detailed models, implies a disk mass of more than 0.05 solar masses, which is enough to form a planetary system like our own.

Detection of the water reservoir in a forming planetary system.

Icy bodies may have delivered the oceans to the early Earth, yet little is known about water in the ice-dominated regions of extrasolar planet-forming disks. The Heterodyne Instrument for the Far-Infrared on board the Herschel Space Observatory has detected emission lines from both spin isomers of cold water vapor from the disk around the young star TW Hydrae. This water vapor likely originates from ice-coated solids near the disk surface, hinting at a water ice reservoir equivalent to several thousand Earth oceans in mass. The water's ortho-to-para ratio falls well below that of solar system comets, suggesting that comets contain heterogeneous ice mixtures collected across the entire solar nebula during the early stages of planetary birth.

Visually and posturally mediated tilt illusion in Parkinson's disease and in labyrinthine defective subjects.

We tested 24 normal subjects, 24 patients with idiopathic Parkinson's disease (PD), and eight patients with bilateral absence of vestibular function (labyrinthine defective [LD] subjects) in their ability to set a straight line to the perceived gravitational vertical (visual vertical). Measurements were taken in static conditions, sitting upright, and lying down on the right side, and during visual background motion at constant angular velocities around the line of sight (roll-motion) in both sitting upright and sideways position. Aims of the study were to determine if the reported increased "visual dependence" in PD was present in a psychophysical task that is independent of motor performance, and to examine the interaction between visual motion and proprioceptive cues in the perception of verticality, in the absence of vestibular function. LD patients showed abnormally large deviations of the visual vertical induced both by lateral body tilt and by visual roll-motion. This suggests that vestibular cues play a significant part in counterbalancing visually and proprioceptive mediated biases on the perception of verticality. In contrast, PD patients were normal in all these tasks indicating that visual dependence in PD is not present at an afferent/perceptual level.

Tremors.

In this article, normal tremor and common types of pathologic tremors seen in the elderly are defined and described along with a review of current treatments. Problems of differential diagnosis are emphasized.

Psychogenic tremors.

We diagnosed 24 patients, 9 men and 15 women ranging in age from 15 to 78 years, with clinically established or documented psychogenic tremors. Clinical presentations were unique, with complex tremors (often resting, postural, and kinetic), unusual temporal profiles (abrupt onset with a variable course), absence of other neurologic signs, inconsistent and incongruous symptomatology, selective disability with ability to perform some functions despite severe tremors, distractibility that lessens or abolishes tremor, atypical tremorgraphic recordings with changing amplitude and frequency, unusual handwriting and drawing specimens, presence of multiple undiagnosed somatizations, unresponsiveness to all treatments, absence of documented disease by laboratory or radiographic tests, presence of psychiatric disease, spontaneous remissions, or recovery with psychotherapy. We present criteria for the diagnosis of psychogenic tremor.

Post-traumatic tremor.

We present seven cases of tremor caused by mild head injury without loss of consciousness. The interval between head trauma to onset of symptoms was 1 to 4 weeks. A posture and kinetic tremor of the hands and head occurred unassociated with other neurologic signs. Myoclonic-like jerking was frequently present. Neuroimaging studies were normal. Clonazepam administration resulted in tremor reduction in three patients and propranolol decreased tremor in one patient. A tremor, similar to essential tremor, can be a rare complication of head trauma.

Lack of association between essential tremor and Parkinson's disease.

Four aspects of a possible association between essential tremor (ET) and Parkinson's disease (PD) were investigated: (1) the frequency of extrapyramidal signs in ET; (2) the frequency of concurrent ET and PD (i.e., monosymptomatic postural tremor for 5 or more years prior to onset of PD); (3) the frequency of ET in the families of PD patients; and (4) the frequency of PD in families of ET patients. Two hundred and thirty-seven ET patients (137 in London and 100 in Chicago) were evaluated. One hundred patients with PD and 100 normal control subjects were also investigated. Mild extrapyramidal signs occurred in only 4.5% of ET patients and were consistent with those found in normal aging. Only 3% of PD patients gave a history of uncomplicated ET. There was no difference in the frequency of a family history of PD between ET patients, a group of PD patients, and control subjects. Frequency of a family history of ET was higher among PD patients than control subjects, although the difference was not statistically significant. These data indicate that there is no association or genetic link between ET and PD.

Is there a central nervous withdrawal syndrome associated with discontinuing long-term treatment with propranolol?

Thirty healthy volunteers were treated with beta-adrenoceptor blocking doses of long-acting propranolol for at least 28 days before being randomized to continue propranolol treatment, receive identical placebo under double-blind conditions, or discontinue all treatment. No evidence of a central nervous withdrawal syndrome occurred during the next 28 days as assessed by changes in psychomotor tests, rating scales, visual analogue scales, tremor recordings and melatonin excretion. Three subjects in the placebo withdrawal group but none in the propranolol group complained of insomnia for up to 14 days of the withdrawal period.

Propranolol and propranolol-LA in essential tremor: a double blind comparative study.

In a double blind, comparative study with 15 patients, a long-acting formulation of propranolol taken once daily (at doses of 160, 240 and 320 mg), was shown to be as effective as conventional propranolol (80 mg three times daily) in reducing the amplitude of essential tremor. The specific protocol employed demonstrated problems inherent in chronic pharmacological trials in essential tremor which have implications for future studies.

Variability in amplitude of untreated essential tremor.

Tremor amplitude was monitored in untreated patients with essential tremor by a procedure designed to minimise fluctuations due to factors known to influence tremor. Tremor was measured hourly for 6 hours in 14 patients and on five to seven occasions (separated by at least a week) in 22 patients. A wide range of within-patient variability was found, with some patients showing little and others showing considerable fluctuations in amplitude over the series of recordings. There was no simple relationship between variability and mean amplitude, frequency, age, duration of tremor or response to a single oral dose of propranolol. Variability did not differ between males and females or between patients with and without a family history of tremor. Diurnal fluctuations showed no consistent pattern. Long-term assessment revealed a significant trend towards lower amplitudes on successive occasions. To overcome systematic bias in data collection, clinical trials in essential tremor should employ large numbers of patients and rigorous randomisation of treatments in cross-over designs.