Short Gas Dissipation Timescales: Diskless Stars in Taurus and Chamaeleon I

We present an Advanced Camera for Surveys/ Solar Blind Channel far-ultraviolet (FUV) study of \h2 gas in 12 weak T Tauri stars in nearby star-forming regions. The sample consists of sources which have no evidence of inner disk dust. Our new FUV spectra show that in addition to the dust, the gas is depleted from the inner disk. This sample is combined with a larger FUV sample of accretors and non-accretors with ages between 1 and 100 Myr, showing that as early as 1--3 Myr, systems both with and without gas are found. Possible mechanisms for depleting gas quickly include viscous evolution, planet formation and photoevaporation by stellar radiation fields. Since these mechanisms alone cannot account for the lack of gas at 1--3 Myr, it is likely that the initial conditions (e.g. initial disk mass or core angular momentum) contribute to the variety of disks observed at any age. We estimate the angular momentum of a cloud needed for most of the mass to fall very close to the central object and compare this to models of the expected distribution of angular momenta. Up to 20% of cloud cores have low enough angular momenta to form disks with the mass close to the star, which would then accrete quickly; this percentage is similar to the fraction of diskless stars in the youngest star forming regions. With our sample, we characterize the chromospheric contribution to the FUV luminosity and find that $L_{FUV}/L_{bol}$ saturates at $\sim10^{-4.1}$.Comment: 5 pages, 4 figures, Accepted to ApJ

Effects of five years of frequent N additions, with or without acidity, on the growth and below-ground dynamics of a young Sitka spruce stand growing on an acid peat: implications for sustainability

International audienceA field manipulation study was established to demonstrate effects of simulated wet N and S deposition on a young (planted 1986) stand of Sitka spruce growing on a predominantly organic soil in an area of low (8?10 kg N ha-1 yr-1) background N deposition in the Scottish borders. From 1996, treatments (six) were applied to the canopies of ten-tree plots in each of four blocks. N was provided as NH4NO3, either with H2SO4 (pH 2.5) at 48 or 96 kg N ha-1 yr-1 inputs or without, at 48 kg N ha-1 yr-1 along with wet (rain water) and dry controls (scaffolding) and a S treatment (Na2SO4). Positive responses (+ >20% over 5 years) with respect to stem area increment were measured in response to N inputs, irrespective of whether acid was included. The positive response to N was not dose related and was achieved against falling base cation concentrations in the foliage, particularly with respect to K. The results suggest young trees are able to buffer the low nutrient levels and produce new growth when there is sufficient N. Inputs of 96 kg N ha-1 yr-1, in addition to ambient N inputs, on this site exceeded tree demand resulting in elevated foliar N, N2O losses and measurable soil water N. These excessive N inputs did not reduce stem area growth. Keywords: acid, canopy application, nitrogen, acid organic soil, simulated wet deposition, soil water, sulphur, young Sitka spruc

Ensemble prediction for nowcasting with a convection-permitting model - II: forecast error statistics

A 24-member ensemble of 1-h high-resolution forecasts over the Southern United Kingdom is used to study short-range forecast error statistics. The initial conditions are found from perturbations from an ensemble transform Kalman filter. Forecasts from this system are assumed to lie within the bounds of forecast error of an operational forecast system. Although noisy, this system is capable of producing physically reasonable statistics which are analysed and compared to statistics implied from a variational assimilation system. The variances for temperature errors for instance show structures that reflect convective activity. Some variables, notably potential temperature and specific humidity perturbations, have autocorrelation functions that deviate from 3-D isotropy at the convective-scale (horizontal scales less than 10 km). Other variables, notably the velocity potential for horizontal divergence perturbations, maintain 3-D isotropy at all scales. Geostrophic and hydrostatic balances are studied by examining correlations between terms in the divergence and vertical momentum equations respectively. Both balances are found to decay as the horizontal scale decreases. It is estimated that geostrophic balance becomes less important at scales smaller than 75 km, and hydrostatic balance becomes less important at scales smaller than 35 km, although more work is required to validate these findings. The implications of these results for high-resolution data assimilation are discussed

The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption

We exploit the high sensitivity and moderate spectral resolution of the $HST$-Cosmic Origins Spectrograph to detect far-ultraviolet spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. We measure a CO column density and rotational excitation temperature of N(CO) = 2 +/- 1 $\times$ 10$^{17}$ cm$^{-2}$ and T_rot(CO) 500 +/- 200 K for the absorbing gas. We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by ultraviolet line photons, predominantly HI LyA. All three objects show emission from CO bands at $\lambda$ $>$ 1560 \AA, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H$_{2}$, and photo-excited H2; all of which appeared as a "continuum" whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar LyA emission profile. We find CO parameters in the range: N(CO) 10$^{18-19}$ cm$^{-2}$, T_{rot}(CO) > 300 K for the LyA-pumped emission. We combine these results with recent work on photo- and collisionally-excited H$_{2}$ emission, concluding that the observations of ultraviolet-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio in the inner disk is ~1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.Comment: 12 pages, 7 figures, 3 tables. ApJ - accepte

The Sparsest Clusters With O Stars

There is much debate on how high-mass star formation varies with environment, and whether the sparsest star-forming environments are capable of forming massive stars. To address this issue, we have observed eight apparently isolated OB stars in the SMC using HST's Advanced Camera for Surveys. Five of these objects appear as isolated stars, two of which are confirmed to be runaways. The remaining three objects are found to exist in sparse clusters, with <10 companion stars revealed, having masses of 1-4 solar mass. Stochastic effects dominate in these sparse clusters, so we perform Monte Carlo simulations to explore how our observations fit within the framework of empirical, galactic cluster properties. We generate clusters using a simplistic -2 power-law distribution for either the number of stars per cluster (N_*) or cluster mass (M_cl). These clusters are then populated with stars randomly chosen from a Kroupa IMF. We find that simulations with cluster lower-mass limits of M_cl,lo >20 solar mass and N_*,lo >40 match best with observations of SMC and Galactic OB star populations. We examine the mass ratio of the second-most massive and most massive stars (m_max,2/m_max), finding that our observations all exist below the 20th percentile of our simulated clusters. However, all of our observed clusters lie within the parameter space spanned by the simulated clusters, although some are in the lowest 5th percentile frequency. These results suggest that clusters are built stochastically by randomly sampling stars from a universal IMF with a fixed stellar upper-mass limit. In particular, we see no evidence to suggest a m_max - M_cl relation. Our results may be more consistent with core accretion models of star formation than with competitive accretion models, and they are inconsistent with the proposed steepening of the integrated galaxy IMF (IGIMF).Comment: 19 pages, 12 figures, accepted for publication in Ap

Streamer Wave Events Observed in Solar Cycle 23

In this paper we conduct a data survey searching for well-defined streamer wave events observed by the Large Angle and Spectrometric Coronagraph (LASCO) on-board the Solar and Heliospheric Observatory (SOHO) throughout Solar Cycle 23. As a result, 8 candidate events are found and presented here. We compare different events and find that in most of them the driving CMEs ejecta are characterized by a high speed and a wide angular span, and the CME-streamer interactions occur generally along the flank of the streamer structure at an altitude no higher than the bottom of the field of view of LASCO C2. In addition, all front-side CMEs have accompanying flares. These common observational features shed light on the excitation conditions of streamer wave events. We also conduct a further analysis on one specific streamer wave event on 5 June 2003. The heliocentric distances of 4 wave troughs/crests at various exposure times are determined; they are then used to deduce the wave properties like period, wavelength, and phase speeds. It is found that both the period and wavelength increase gradually with the wave propagation along the streamer plasma sheet, and the phase speed of the preceding wave is generally faster than that of the trailing ones. The associated coronal seismological study yields the radial profiles of the Alfv\'en speed and magnetic field strength in the region surrounding the streamer plasma sheet. Both quantities show a general declining trend with time. This is interpreted as an observational manifestation of the recovering process of the CME-disturbed corona. It is also found that the Alfv\'enic critical point is at about 10 R$_\odot$ where the flow speed, which equals the Alfv\'en speed, is $\sim$ 200 km s$^{-1}$