Over the past decade, ice loss from the Greenland Ice Sheet increased as a result of both increased surface melting and ice discharge to the ocean 1,2 . The latter is controlled by acceleration of ice flow and subsequent thinning of fast-flowing marine-terminating outlet glaciers 3 . Quantifying the future dynamic contribution of such glaciers to sea-level rise (SLR) remains a major challenge since outlet-glacier dynamics are poorly understood 4 . Here we present a model that includes a fully dynamic treatment of marine termini. We use this model to

Abstract. We present a high-angular-resolution molecular line and millimeter continuum study of the massive star formation site IRAS 05358+3543. Observations with the Plateau de Bure Interferometer in CO 1-0, SiO 2-1 and H 13 CO + 1-0 reveal at least three outflows which cannot be separated in single-dish data. Observations at millimeter and sub-millimeter wavelengths from the IRAM 30 m telescope and the CSO provide additional information on the region. The most remarkable feature is a highly collimated (collimation factor ∼10) and massive (>10 M ) bipolar outflow of ∼1 pc length, which is part of a quadrupolar outflow system. The three observed molecular outflows forming the IRAS 05358+3543 outflow system resemble, in structure and collimation, those typical of low-mass star-forming regions. They might therefore, just like low-mass outflows, be explained by shock entrainment models of jets. We estimate a mass accretion rate of ∼10 −4 M /yr, sufficient to overcome the radiative pressure of the central object and to build up a massive star, lending further support to the hypothesis that massive star formation occurs similarly to low-mass star formation, only with higher accretion rates and energetics. In the millimeter continuum, we find three sources near the center of the quadrupolar outflow, each with a mass of 75-100 M . These cores are associated with a complex region of infrared reflection nebulosities and their embedded illuminating sources. The molecular line data show that SiO is found mostly in the outflows, whereas H 13 CO + traces core-like structures, though likely with varying relative abundances. Thermal CH3OH comprises both features and can be disentangled into a core-tracing component at the line center, and wing emission following the outflows. A CO line-ratio study (using data of the J = 1-0, 2-1 and 6-5 transitions) reveals local temperature gradients.

ABSTRACT. Physically based projections of the Greenland ice sheet contribution to future sea-level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within the ice flow model itself. Here a higher-order, three-dimensional thermomechanical ice flow model is used, initialized to the present-day geometry. The forcing comes from a high-resolution regional climate model and from a flowline model applied to four individual marine-terminated glaciers, and results are subsequently extended to the entire ice sheet. The experiments span the next 200 years and consider climate scenario SRES A1B. The surface mass-balance (SMB) scheme is taken either from a regional climate model or from a positive-degree-day (PDD) model using temperature and precipitation anomalies from the underlying climate models. Our model results show that outlet glacier dynamics only account for 6-18% of the sea-level contribution after 200 years, confirming earlier findings that stress the dominant effect of SMB changes. Furthermore, interaction between SMB and ice discharge limits the importance of outlet glacier dynamics with increasing atmospheric forcing. Forcing from the regional climate model produces a 14-31% higher sea-level contribution compared to a PDD model run with the same parameters as for IPCC AR4.

Geodetic observations show several large, sudden increases in flow speed at Helheim Glacier, one of Greenland's largest outlet glaciers, during summer, 2007. These step‐like accelerations, detected along the length of the glacier, coincide with teleseismically detected glacial earthquakes and major iceberg calving events. No coseismic offset in the position of the glacier surface is observed; instead, modest tsunamis associated with the glacial earthquakes implicate glacier calving in the seismogenic process. Our results link changes in glacier velocity directly to calving‐front behavior at Greenland's largest outlet glaciers, on timescales as short as minutes to hours, and clarify the mechanism by which glacial earthquakes occur.

Using observations obtained with the Wide-Field Camera 3 on board the Hubble Space Telescope, we have studied the properties of the stellar populations in the central regions of 30 Dor in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterize and quantify this effect using young massive main-sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main-sequence (PMS) stars by looking for objects with a strong (>4σ ) H α excess emission and find about 1150 of them over the entire field. Comparison of their location in the Hertzsprung-Russell diagram with theoretical PMS evolutionary tracks for the appropriate metallicity reveals that about one-third of these objects are younger than ∼4 Myr, compatible with the age of the massive stars in the central ionizing cluster R 136, whereas the rest have ages up to ∼30 Myr, with a median age of ∼12 Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very center of the cluster. We attribute this latter effect to photo-evaporation of the older circumstellar disks caused by the massive ionizing members of R 136.

We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations of the central 56 ′′ ×57 ′′ (14pc×14.25pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar Initial Mass Function (IMF) down to ∼1 M ⊙ in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few Myr. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 M ⊙ and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 M ⊙ for 3 Myr old objects. We find an IMF of dN d log M ∝ M −1.20±0.2 over the mass range 1.1-20 M ⊙ only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 M ⊙ at a distance of 5 pc from the center, in contrast to a flattening at 2 M ⊙ at a radius of 2 pc, reported in a previous optical HST study. flattening at 2 M ⊙ at a radius of 2 pc previously found. We examine several possible reasons for the different results including the possible presence of mass segregation and the effects of differential extinction, particularly for the pre-main sequence sources. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.

With Spitzer IRS, we have obtained sensitive low-resolution spectroscopy from 5 to 35 μm for six supernova remnants (SNRs) that show evidence of shocked molecular gas: Kes 69, 3C 396, Kes 17, G346.6−0.2, G348.5−0.0, and G349.7+0.2. Bright pure rotational lines of molecular hydrogen are detected at the shock front in all remnants, indicative of radiative cooling from shocks interacting with dense clouds. We find the excitation of H 2 S(0)-S(7) lines in these SNRs requires two nondissociative shock components: a slow 10 km s −1 C-shock through clumps of density 10 6 cm −3 , and a faster 40-70 km s −1 C-shock through a medium of density 10 4 cm −3 . The ortho-to-para ratio for H 2 in the warm shocked gas is typically found to be much less than the LTE value, suggesting that these SNRs are propagating into cold quiescent clouds. Additionally, a total of 13 atomic fine-structure transitions of Ar + , Ar ++ , Fe + , Ne + , Ne ++ , S ++ , and Si + are detected. The ionic emitting regions are spatially segregated from the molecular emitting regions within the IRS slits. The presence of ionic lines with high appearance potential requires the presence of much faster, dissociative shocks through a lower density medium. The IRS slits are sufficiently wide to include regions outside the SNR which permits emission from diffuse gas around the remnants to be separated from the shocked emission. We find the diffuse H 2 gas projected outside the SNR is excited to a temperature of 100-300 K with a warm gas fraction of at least 0.5%-15% along the line of sight.

We present a combined analysis of the low-mass initial mass function (IMF) for seven star-forming regions. We first demonstrate that the ratios of stars to brown dwarfs are consistent with a single underlying IMF. By assuming that the underlying IMF is the same for all seven clusters and by combining the ratio of stars to brown dwarfs from each cluster, we constrain the shape of the brown dwarf IMF and find it to be consistent with a lognormal IMF. This provides the strongest constraint yet that the substellar IMF turns over ( ,