Episodic Outbursts Research Articles

LUMINOSITY OUTBURSTS IN INTERACTING PROTOPLANETARY SYSTEMS

FU Orionis type objects (fuors) are characterized by rapid (tens to hundreds years) episodic outbursts, during which the luminosity increases by orders of magnitude. One of the possible causes of such events is a close encounter between stars and protoplanetary disks. Numerical simulations show that the fuor-like outburst ignition requires a very close encounter ranging from a few to a few tens of au. In contrast, the observed stellar objects in fuor binaries are usually hundreds of au apart. Simple mathematical estimates show that if such a close approach took place, the binary stellar components would have an unrealistic relative velocity, at least an order of magnitude greater than the observed velocity dispersion in young stellar clusters. Thus, the bursts are either triggered with a certain delay after passage of the periastron or their ignition does not necessary require a close encounter and hence the outburst is not caused by the primordial gravitational perturbation of the protoplanetary disk. In this work, an encounter of a star surrounded by a protoplanetary disk with a diskless external stellar object was modeled using numerical hydrodynamics simulations. We showed that even fly-bys with a relatively large periastron (at least 500 au) can result in fuor-like outbursts. Moreover, the delay between the periastron passage and the burst ignition can reach several kyr. It was shown for the first time by means of numerical modeling that the perturbation of the disk caused by the external object can trigger a cascade process, which includes the development of the thermal instability in the innermost disk followed by the magneto-rotational instability ignition. Because of the sequential development of these instabilities, the rapid increase in the accretion rate occurs, resulting in the luminosity increase by more than two orders of ma-gnitude.

Primordial dusty rings and episodic outbursts in protoplanetary discs

ABSTRACT We investigate the formation and evolution of ‘primordial’ dusty rings occurring in the inner regions of protoplanetary discs, with the help of long-term, coupled dust-gas, magnetohydrodynamic simulations. The simulations are global and start from the collapse phase of the parent cloud core, while the dead zone is calculated via an adaptive α formulation by taking into account the local ionization balance. The evolution of the dusty component includes its growth and back reaction on to the gas. Previously, using simulations with only a gas component, we showed that dynamical rings form at the inner edge of the dead zone. We find that when dust evolution, as well as magnetic field evolution in the flux-freezing limit are included, the dusty rings formed are more numerous and span a larger radial extent in the inner disc, while the dead zone is more robust and persists for a much longer time. We show that these dynamical rings concentrate enough dust mass to become streaming unstable, which should result in a rapid planetesimal formation even in the embedded phases of the system. The episodic outbursts caused by the magnetorotational instability have a significant impact on the evolution of the rings. The outbursts drain the inner disc of grown dust, however, the period between bursts is sufficiently long for the planetesimal growth via streaming instability. The dust mass contained within the rings is large enough to ultimately produce planetary systems with the core accretion scenario. The low-mass systems rarely undergo outbursts, and, thus, the conditions around such stars can be especially conducive for planet formation.

Eruptive Behavior of Magnetically Layered Protoplanetary Disks in Low-metallicity Environments

Abstract A protoplanetary disk (PPD) typically forms a dead zone near its midplane at a distance of a few astronomical units from the central protostar. Accretion through such a magnetically layered disk can be intrinsically unstable and has been associated with episodic outbursts in young stellar objects. We present the first investigation into the effects of a low-metallicity environment on the structure of the dead zone, as well as the resulting outbursting behavior of the PPD. We conducted global numerical hydrodynamic simulations of PPD formation and evolution in the thin-disk limit. The consequences of metallicity were considered via its effects on the gas and dust opacity of the disk, the thickness of the magnetically active surface layer, and the temperature of the prestellar cloud core. We show that the metal-poor disks accumulate much more mass in the innermost regions as compared to the solar-metallicity counterparts. The duration of the outbursting phase also varies with metallicity; the low-metallicity disks showed more powerful luminosity eruptions with a shorter burst phase, which was confined mostly to the early, embedded stages of the disk evolution. The lowest-metallicity disks with the higher cloud core temperature showed the most significant differences. The occurrence of outbursts was relatively rare in the disks around low-mass stars, and this was especially true at the lowest metallicities. We conclude that the metal content of the disk environment can have profound effects on both the disk structure and evolution in terms of episodic accretion.

Seismic characterization of a rapidly-rising jökulhlaup cycle at the A.P. Olsen Ice Cap, NE-Greenland

AbstractRapidly-rising jökulhlaups, or glacial outburst floods, are a phenomenon with a high potential for damage. The initiation and propagation processes of a rapidly-rising jökulhlaup are still not fully understood. Seismic monitoring can contribute to an improved process understanding, but comprehensive long-term seismic monitoring campaigns capturing the dynamics of a rapidly-rising jökulhlaup have not been reported so far. To fill this gap, we installed a seismic network at the marginal, ice-dammed lake of the A.P. Olsen Ice Cap (APO) in NE-Greenland. Episodic outbursts from the lake cause flood waves in the Zackenberg river, characterized by a rapid discharge increase within a few hours. Our 6 months long seismic dataset comprises the whole fill-and-drain cycle of the ice-dammed lake in 2012 and includes one of the most destructive floods recorded so far for the Zackenberg river. Seismic event detection and localization reveals abundant surface crevassing and correlates with changes of the river discharge. Seismic interferometry suggests the existence of a thin basal sedimentary layer. We show that the ballistic part of the first surface waves can potentially be used to infer medium changes in both the ice body and the basal layer. Interpretation of time-lapse interferograms is challenged by a varying ambient noise source distribution.

Episodic formation of refractory inclusions in the Solar System and their presolar heritage

Refractory inclusions [Ca-Al-rich Inclusions (CAIs) and Amoeboid Olivine Aggregates (AOAs)] in primitive meteorites are the oldest Solar System solids. They formed in the hot inner protoplanetary disk and, as such, provide insights into the earliest disk dynamics and physicochemical processing of the dust and gas that accreted to form the Sun and its planetary system. Using the short-lived 26Al to 26Mg decay system, we show that bulk refractory inclusions in CV (Vigarano-type) and CR (Renazzo-type) carbonaceous chondrites captured at least two distinct 26Al-rich (26Al/27Al ratios of ∼5 × 10−5) populations of refractory inclusions characterized by different initial 26Mg/24Mg isotope compositions (μ26Mg*0). Another 26Al-poor CAI records an even larger μ26Mg*0 deficit. This suggests that formation of refractory inclusions was punctuated and recurrent, possibly associated with episodic outbursts from the accreting proto-Sun lasting as short as <8000 yr. Our results support a model in which refractory inclusions formed close to the hot proto-Sun and were subsequently redistributed to the outer disk, beyond the orbit of Jupiter, plausibly via stellar outflows with progressively decreasing transport efficiency. We show that the magnesium isotope signatures in refractory inclusions mirrors the presolar grain record, demonstrating a mutual exclusivity between 26Al enrichments and large nucleosynthetic Mg isotope effects. This suggests that refractory inclusions formed by incomplete thermal processing of presolar dust, thereby inheriting a diluted signature of their isotope systematics. As such, they record snapshots in the progressive sublimation of isotopically anomalous presolar carriers through selective thermal processing of young dust components from the proto-Solar molecular cloud. We infer that 26Al-rich refractory inclusions incorporated 26Al-rich dust which formed <5 Myr prior to our Sun, whereas 26Al-poor inclusions (such as FUN- and PLAC-type CAIs) incorporated >10 Myr old dust.

An Episodic Wide-angle Outflow in HH 46/47

Abstract During star formation, the accretion disk drives fast MHD winds, which usually contain two components, a collimated jet and a radially distributed wide-angle wind. These winds entrain the surrounding ambient gas producing molecular outflows. We report a recent observation of 12CO (2–1) emission of the HH 46/47 molecular outflow by the Atacama Large Millimeter/submillimeter Array, in which we identify multiple wide-angle outflowing shell structures in both the blueshifted and redshifted outflow lobes. These shells are highly coherent in position–position–velocity space, extending to ≳40–50 km s−1 in velocity and 104 au in space, with well-defined morphology and kinematics. We suggest these outflowing shells are the result of the entrainment of ambient gas by a series of outbursts from an intermittent wide-angle wind. Episodic outbursts in collimated jets are commonly observed, yet detection of a similar behavior in wide-angle winds has been elusive. Here we show clear evidence that the wide-angle component of the HH 46/47 protostellar outflows experiences variability similar to that seen in the collimated component.

Characterisation of short-term extreme methane fluxes related to non-turbulent mixing above an Arctic permafrost ecosystem

Abstract. Methane (CH4) emissions from biogenic sources, such as Arctic permafrost wetlands, are associated with large uncertainties because of the high variability of fluxes in both space and time. This variability poses a challenge to monitoring CH4 fluxes with the eddy covariance (EC) technique, because this approach requires stationary signals from spatially homogeneous sources. Episodic outbursts of CH4 emissions, i.e. triggered by spontaneous outgassing of bubbles or venting of methane-rich air from lower levels due to shifts in atmospheric conditions, are particularly challenging to quantify. Such events typically last for only a few minutes, which is much shorter than the common averaging interval for EC (30 min). The steady-state assumption is jeopardised, which potentially leads to a non-negligible bias in the CH4 flux. Based on data from Chersky, NE Siberia, we tested and evaluated a flux calculation method based on wavelet analysis, which, in contrast to regular EC data processing, does not require steady-state conditions and is allowed to obtain fluxes over averaging periods as short as 1 min. Statistics on meteorological conditions before, during, and after the detected events revealed that it is atmospheric mixing that triggered such events rather than CH4 emission from the soil. By investigating individual events in more detail, we identified a potential influence of various mesoscale processes like gravity waves, low-level jets, weather fronts passing the site, and cold-air advection from a nearby mountain ridge as the dominating processes. The occurrence of extreme CH4 flux events over the summer season followed a seasonal course with a maximum in early August, which is strongly correlated with the maximum soil temperature. Overall, our findings demonstrate that wavelet analysis is a powerful method for resolving highly variable flux events on the order of minutes, and can therefore support the evaluation of EC flux data quality under non-steady-state conditions.

Bow shocks in water fountain jets

AbstractWe briefly introduce the VLBI maser astrometric analysis of IRAS 18043–2116 and IRAS 18113–2503, two remarkable and unusual water fountains with spectacular bipolar bow shocks in their high-speed collimated jet-driven outflows. The 22 GHz H2O maser structures and velocities clearly show that the jets are formed in very short-lived, episodic outbursts, which may indicate episodic accretion in an underlying binary system.

The JCMT Transient Survey: Data Reduction and Calibration Methods

Abstract Though there has been a significant amount of work investigating the early stages of low-mass star formation in recent years, the evolution of the mass assembly rate onto the central protostar remains largely unconstrained. Examining in depth the variation in this rate is critical to understanding the physics of star formation. Instabilities in the outer and inner circumstellar disk can lead to episodic outbursts. Observing these brightness variations at infrared or submillimeter wavelengths constrains the current accretion models. The JCMT Transient Survey is a three-year project dedicated to studying the continuum variability of deeply embedded protostars in eight nearby star-forming regions at a one-month cadence. We use the SCUBA-2 instrument to simultaneously observe these regions at wavelengths of 450 and 850 μm. In this paper, we present the data reduction techniques, image alignment procedures, and relative flux calibration methods for 850 μm data. We compare the properties and locations of bright, compact emission sources fitted with Gaussians over time. Doing so, we achieve a spatial alignment of better than 1″ between the repeated observations and an uncertainty of 2%–3% in the relative peak brightness of significant, localized emission. This combination of imaging performance is unprecedented in ground-based, single-dish submillimeter observations. Finally, we identify a few sources that show possible and confirmed brightness variations. These sources will be closely monitored and presented in further detail in additional studies throughout the duration of the survey.

NGC 5195 IN M51: FEEDBACK “BURPS” AFTER A MASSIVE MEAL?

ABSTRACT We describe a double-arc-like X-ray structure lying ∼15″–30″ (∼0.8–1.7 kpc) south of the NGC 5195 nucleus, visible in the merged exposures of long Chandra pointings of M51. The curvature and orientation of the arcs argues for a nuclear origin. The arcs are radially separated by ∼15″ (∼1 kpc), but are rotated relative to each other by ∼30°. From an archival image, we find a slender Hα-emitting region just outside the outer edge of the outer X-ray arc, suggesting that the X-ray-emitting gas plowed up and displaced the Hα-emitting material from the galaxy core. Star formation may have commenced in that arc. Hα emission is present at the inner arc, but appears more complex in structure. In contrast to an explosion expected to be azimuthally symmetric, the X-ray arcs suggest a focused outflow. We interpret the arcs as episodic outbursts from the central super-massive black hole (SMBH). We conclude that NGC 5195 represents the nearest galaxy exhibiting on-going, large-scale outflows of gas, in particular, two episodes of a focused outburst of the SMBH. The arcs represent a clear demonstration of feedback.

Theorising Alcohol in Public Discourse: Moral Panics or Moral Regulation?

In recent years, politicians, police chiefs, journalists and medical professionals have been vocal in their condemnation of excessive alcohol consumption. Newspapers routinely refer to a “binge-drinking epidemic”, Prime Minister David Cameron has spoken of drink-induced “mayhem on the streets” and Work and Pensions Secretary Iain Duncan-Smith has claimed alcohol is “damaging the fabric of the nation”. Despite the widespread belief that we live in new and difficult times, the 'drink problem' in Britain is nothing new. From the eighteenth century 'gin craze' and the Victorian temperance movement to the more current debates over twenty-four hour licensing and minimum pricing, acute public concern about drinking has been a regular feature of British history. Are these recurrent anxieties the result of successive episodic outbursts of irrational anxiety about different social changes, as classic moral panic theory might postulate? Or can other theoretical frameworks, particularly moral regulation theory, improve our understanding of the British relationship with alcohol?Various academics have examined the connection of alcohol consumption to social anxiety or moralisation. This paper brings fresh insights in two main ways: firstly by focusing on British history, a neglected area in the sociology of drinking; and secondly, by bringing enquiries up to date with an examination of recent public discourse on drink, which has been largely preoccupied with the idea of alcohol as a health problem. Drawing on the work Alan Hunt and Chas Critcher, this paper uses the history of British concerns about drinking to shed new light on the question of whether moral panic theory is still relevant, and, if so, in what form.

A LONG-LASTING QUIESCENCE PHASE OF THE ERUPTIVE VARIABLE V1118 ORI

V1118 Ori is an eruptive variable belonging to the EXor class of Pre-Main Sequence stars whose episodic outbursts are attributed to disk accretion events. Since 2006, V1118 Ori is in the longest quiescence stage ever observed between two subsequent outbursts of its recent history. We present near-infrared photometry of V1118 Ori carried out during the last eight years, along with a complete spectroscopic coverage from 0.35 to 2.5 um. A longterm sampling of V1118 Ori in quiescence has never been done, hence we can benefit from the current circumstance to determine the lowest values (i.e. the zeroes) of the parameters to be used as a reference for evaluating the physical changes typical of more active phases. A quiescence mass accretion rate between 1--3 $\times$ 10$^{-9}$ M$_{\sun}$ yr$^{-1}$ can be derived and the difference with previous determinations is discussed. From line emission and IR colors analysis a visual extinction of 1-2 mag is consistently derived, confirming that V1118 Ori (at least in quiescence) is a low-extinction T Tauri star with a bolometric luminosity of about 2.1 L$_{\sun}$. An anti-correlation exists between the equivalent width of the emission lines and the underlying continuum. We searched the literature for evaluating whether or not such a behaviour is a common feature of the whole class. The anti-correlation is clearly recognizable for all the available EXors in the optical range (H$\beta$ and H$\alpha$ lines), while it is not as much evident in the infrared (Pa$\beta$ and Br$\gamma$ lines). The observed anti-correlation supports the accretion-driven mechanism as the most likely to account for continuum variations.

DYNAMICAL EVOLUTION OF VISCOUS DISKS AROUND BE STARS. II. POLARIMETRY

Be stars exhibit variability for a great number of observables. Putting the pieces together of the disk dynamics is not an easy task and requires arduous modeling before achieving a good fit of the observational data. In order to guide the modeling process and make it more efficient, it is very instructive to investigate reference dynamical cases. This paper focuses on continuum polarimetric quantities and is the second of a series that aims to demonstrate the capacity of deriving the dynamical history and fundamental parameters of a classical Be star through the follow-up of various observables. After a detailed study of the different opacities at play in the formation of polarized spectra, we investigate predictions of polarimetric observables in the continuum for different dynamical scenarios. Our models are based on a coupling of a hydrodynamic viscous decretion simulations in a disk and a 3-D non-LTE radiative transfer code. Introducing the polarization color diagram (PCD), we show that certain combinations of polarimetric observables exhibit features that are characteristic of a mass loss history. This diagram also enables to estimate fundamental parameters such as the inclination angle, the disk density scale and the $\alpha$ viscous diffusion parameter. We present the PCD as a powerful diagnosis tool to track the dynamical phases of a Be star such as disk building-up, dissipation, periodic and episodic outbursts. Finally we confront our models with observations of 4 Be stars that exhibited long-term polarimetric activity.

Active processes, morphology, and dynamics of icy debris fans: Landform evolution along rapidly degrading escarpments in alpine regions undergoing recent deglaciation

In the past few decades the impacts of climate warming have been significant in alpine glaciated regions. Many valley glaciers formerly linked as distributary glaciers to high-level icecaps have decoupled at their icefalls, exposing major escarpments and generating a suite of dynamic landforms dominated by mass wasting. Ice-dominated landforms, here termed icy debris fans, develop rapidly by ice avalanching, rockfall, and icy debris flow. Field-based reconnaissance studies at two alpine settings, the Wrangell Mountains of Alaska and the Southern Alps of New Zealand, provide a preliminary morphogenetic model of spatial and temporal evolution of icy debris fans in a range of alpine settings. The influence of these processes on landform evolution is largely unrecognized in the literature dealing with post-glacial landform adjustment known as the paraglacial. A better understanding of these dynamic processes will be increasingly important because of the extreme geohazards characterizing these areas.Our field studies show that after glacier decoupling, icy debris fans begin to form along the base of bedrock escarpments at the mouths of catchments and prograde over valley glaciers. The presence of a distinct catchment, apex, and fan morphology distinguishes these landforms from other landforms common in periglacial hillslope settings receiving abundant clastic debris and ice. Ice avalanching is the most abundant process involved in icy debris fan formation. Fans developed below weakly incised catchments are dominated by ice avalanching and are composed primarily of ice with minor lithic detritus. Typically, avalanches fall into the fan catchments where sediments transform into grainflows that flow onto the fans. Once on the fans, avalanche deposits ablate rapidly, flattening and concentrating lithic fragments at the surface. Icy debris fans may become thick enough to become glaciers with splay crevasse systems. Fans developed below larger, more complex catchments are composed of higher proportions of lithic detritus resulting from temporary storage of ice and lithic detritus deposits within the catchment. Episodic outbursts of meltwater from the icecap may mix with the stored sediments and mobilize icy debris flows (mixture of ice and lithic clasts) onto the fans.Our observations indicate that the entire evolutionary cycle of icy debris fans probably occurs during an early paraglacial interval (i.e., decades to 100years). Observations comparing avalanche frequency, volume, and fan morphologic evolution at the Alaska site between 2006 and 2010 illustrate complex response between icy debris fans even within the same cirque — where one fan may be growing while others are downwasting because of differences in ice supply controlled by their respective catchments and icecap contributions. As ice supply from the icecap diminishes through time, icy debris fans rapidly downwaste and eventually evolve into talus cones that receive occasional but ephemeral ice avalanches.

Homesteading in the Age of Plastics

Homesteading in the Age of Plastics Karen R. Merrill (bio) Brian Q. Cannon. Reopening the Frontier: Homesteading in the Modern West. Lawrence: University Press of Kansas, 2009. viii + 306 pp. Notes, bibliography, and index. $39.95. In 1953, over 4,000 people gathered in the center of Rupert, Idaho to see which lucky few among them would be randomly selected and awarded homesteads on the nearby Bureau of Reclamation Minidoka Project. As Brian Q. Cannon describes this and similar events in the West following World War II, festivities surrounded the lottery, and paeans to the American frontier and pioneers abounded in speeches by government officials. With strong preference given to World War II veterans, the federal program to settle homesteaders on Bureau of Reclamation land was compelling on all sorts of fronts—as recompense for the service that soldiers offered in the war, as a way to provide historical continuity with the almost-sacred image of the Western homesteader following the Civil War, and as an effort to put to agricultural use parts of the arid West. As Cannon points out, it was not a large program: in the end, about 3,000 farms were homesteaded on public lands with the help of the Bureau of Reclamation between 1946 and 1966. But he argues that it is worthy of examination because the program appears, at first glance, to stand in such stark contrast to our understandings of postwar American life, with its embrace of mass production in manufacturing and agriculture and large organizational structures. It pays to remember, even if such a reference relies on a cliché of the 1950s, that, by 1953, when the Minidoka Project was open to homesteading, Tupperware parties were in full glory. (By 1958, Earl Tupper could sell his company for $16 million.) In contrast to these suburban plastic-kitchenware-sale socials, the women who settled with their husbands on the homesteads of the postwar West typically did not even have electricity. One woman recounted that, in those early years of homesteading, she and her husband had three children still in diapers and no bathroom or running water in her house. Another woman described her chagrin of homesteading in Washington and discovering the backwardness of the residents in the area. When two “old-timers” came to visit, she recalled years later, “they set a sack of candy bars on the table and they said, ‘This is for the kids.’ And we looked at it and the candy bars had [End Page 706] all gone white. They [were] probably twenty years old’” (p. 108). Clearly, homesteading in the postwar West felt like a trip back in time. The seemingly anomalous place of a federal homesteading program in mid-twentieth century America is part of the appeal for Cannon. As he notes in the introduction: At first, these episodic outbursts of homesteading appear to be anachronistic throwbacks to an earlier era, romantic and entertaining sideshows that curiously brought new lands into production at the same time that the government was paying other farmers to withhold land on existing farms from cultivation. . . . [But] the unfolding of a quintessential element of the Old West—homesteading—within an urbanizing modern West created exceptionally vivid contrasts. Those contrasts, Cannon argues, provide fertile terrain to explore Patricia Nelson Limerick’s thesis—articulated most familiarly in The Legacy of Conquest—that “the continuities in western history override the temporal differences. We can better understand change and continuity in western history by examining the West after World War II through the lens of postwar homesteading” (p. 4). Cannon sees continuity in the ideological appeal of homesteading to many postwar Americans. Whether the postwar homesteaders on the Minidoka Project and other reclamation homesteading projects had come from a rural past or not, the allure of owning and working the land and of raising a family on a farm, rooted in traditional rural values, was great. However, whereas these values dominated American views of the West through the nineteenth and early twentieth centuries, they now circulated in a changing cultural landscape in the postwar period, as many Americans increasingly understood Western lands as places where recreational, aesthetic, and/or environmental values should reign. In the end...

Treatment of Tics and Tourette Syndrome

Tics come in a variety of types and frequencies; have a waxing and waning course; are exacerbated by stress, anxiety, and fatigue; and often resolve or improve in the teenage or early adult years. Tourette syndrome requires the presence of chronic, fluctuating motor and phonic tics. In addition to tics, individuals with Tourette syndrome often have a variety of comorbid conditions such as attention deficit hyperactivity disorder (ADHD), obsessive-compulsive disorder, depression and anxiety, episodic outbursts, and academic difficulties. These conditions often are a greater source of difficulty than the tics themselves. All patients with tics should be evaluated to assure proper diagnosis and to identify any associated psychopathology or academic difficulty. The treatment of tics begins with education of the patient and family, including discussions about the fundamentals of tics: their characteristics, etiology, outcomes, and available treatments. Therapy should be individualized based on the extent of impairment, available support, ability to cope, and the presence of other problems. Indications for the treatment of tics include psychosocial problems (loss of self-esteem, comments from peers, excessive worries about tics, diminished participation in activities), functional difficulties, classroom disruption, and physical discomfort. A variety of behavioral approaches can be used. Recent studies have emphasized the value of comprehensive behavioral intervention for tics (CBIT). Because habit reversal is the major component of CBIT, a cooperative patient, the presence of a premonitory urge, and a committed family are essential ingredients for success. If tic-suppressing medication is required, a two-tier approach and monotherapy are recommended. First-tier medications, notably the α-adrenergic agonists, are recommended for individuals with milder tics, especially persons with both tics and ADHD. Second-tier medications include various typical and atypical neuroleptics. Their sequence of prescription is often based on physician experience; I favor pimozide and fluphenazine. Atypical antipsychotics, such as risperidone and aripiprazole, have some advantages based on their side-effect profile and are particularly beneficial in individuals with significant co-existing behavioral issues. As will become readily apparent, however, few medications have been adequately assessed. Deep brain stimulation is an emerging therapy, but further data are required to optimize the location of electrode placement and stimulation and to determine precise indications for its implementation. Stimulant medication is effective in treating ADHD in children with tics; studies reducing concerns about its use are discussed.

Episodic accretion on to strongly magnetic stars

Some accreting neutron stars and young stars show unexplained episodic flares in the form of quasi-periodic oscillations or recurrent outbursts. In a series of two papers we present new work on an instability that can lead to episodic outbursts when the accretion disc is truncated by the star's strong magnetic field close to the corotation radius (where the Keplerian frequency matches the star's rotational frequency). In this paper we outline the physics of the instability and use a simple parameterization of the disc-field interaction to explore the instability numerically, which we show can lead to repeated bursts of accretion as well as steady-state solutions, as first suggested by Sunyaev and Shakura. The cycle time of these bursts increases with decreasing accretion rate. These solutions show that the usually assumed `propeller' state, in which mass is ejected from the system, does not need to occur even at very low accretion rates.

Inside Out; Outside In

C omets harbor ice and organic material that formed early in our solar system and was partly incorporated into the planets, including ours. They often appear to be huge in the sky, but this is deceptive—their visible coma is produced from a relatively small nucleus, typically a few tens of kilometers across or less. Their small size may have helped to preserve a pristine record of early solar system chemistry. On 3 July 2005, Deep Impact penetrated the nucleus of comet 9P/Tempel 1, ejecting dust and debris from inside the nucleus that was studied by a nearby spacecraft and dozens of other telescopes and spacecraft worldwide. Results from the observations of this truly international experiment (originally published online on 8 and 15 September 2005) are described in six papers in this special section. Our solar system has two populations of comets: One now resides in a region near and just beyond the orbit of Pluto (the Kuiper Belt), and one group of these (including comet 9P/Tempel 1) gets scattered into orbits that bring them closer to the Sun and Earth; another population now resides much farther out in the Oort Cloud and includes many of our famous comets (Halley, for example). Although now far distant, it is thought that the Oort comets originated near Neptune and Uranus—closer in than the Kuiper Belt comets—but were flung outward (many may have left our solar system entirely) as these and the other giant planets formed. ![Figure][1] CREDIT: NASA/JPL/UNIVERSITY OF MARYLAND As discussed in these papers, the Deep Impact experiment revealed several surprises about the nature of the nucleus of Tempel 1 and the origin of comets (see also the News story by Kerr in the 9 September 2005 issue of Science ). On approach, Deep Impact found that the nucleus contained regions with different layering, perhaps suggesting the amalgamation of two or more separate pieces. Observations from the supporting telescopes, and later from the spacecraft, revealed several episodic outbursts of material from the nucleus. Its surface also contained impact craters, the first seen on a cometary nucleus. Data from the impact itself imply that the nucleus was loosely consolidated and had a low density, consistent with porous ice. Clear evidence of some complex internal layering in the nucleus was not evident. Together these observations reveal a complex history and help tie in features of the nucleus to the formation of its coma. Analysis of the debris cone identified organic molecules, water ice, and silicate-rich dust. Interestingly, the composition and relative abundance of these gases and particles in this Kuiper Belt comet are similar to those of several Oort comets previously studied. If so, then at least some Kuiper Belt comets may have originated close to or within the giant-planet region, implying a more common origin for now-distant comets and suggesting interesting dynamics within this region of the early solar system. [1]: pending:yes

Major Δ 14C excursions during the late glacial and early Holocene: changes in ocean ventilation or solar forcing of climate change?

The atmospheric 14 C record during the Late Glacial and the early Holocene shows sharp increases simultaneous with cold climatic phases. These increases in the atmospheric 14 C content are usually explained as the effect of reduced oceanic CO 2 ventilation after episodic outbursts of large meltwater reservoirs into the North Atlantic. In this hypothesis the stagnation of the thermohaline circulation is the cause of both climate change as well as an increase in atmospheric 14 C . As an alternative hypothesis we propose that changes in 14 C production give an indication for the cause of the recorded climate shifts: changes in solar activity cause fluctuations in the solar wind, which modulate the cosmic ray intensity and related 14 C production. Two possible mechanisms amplifying the changes in solar activity may result in climate change. In the case of a temporary decline in solar activity: (1) reduced solar UV intensity may cause a decline of stratospheric ozone production and cooling as a result of less absorption of sunlight. This might influence atmospheric circulation patterns (extension of Polar Cells and equatorward relocation of mid-latitude storm tracks), with effects on oceanic circulation, and (2) increased cosmic ray intensity may stimulate cloud formation and precipitation, while 14 C production increases.

A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles.

To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (<1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1-8 Myr) and early Permian (275-290 Myr) times. But for most of the Mesozoic era (65-250 Myr), CO2 levels were high (1,000-2,000 p.p.m.v.), with transient excursions to even higher CO2 (>2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records, but suggest that CO2 reconstructions based on carbon isotope proxies may be compromised by episodic outbursts of isotopically light methane. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr.