94 research outputs found
From the Top to the Bottom of the Main Sequence: A Complete Mass Function of the Young Open Cluster M35
We present very deep and accurate photometry of the open cluster M35 (VRIc
filters). We have covered a region of 27.5x27.5 square arcmin.
The data range from Ic=12.5 to 23.5 mag, and the color intervals are
0.4\le(V-I)c\le3.0, 0.5\le(R-I)c\le2.5. Roughly, these values span from 1.6
M_\odot down to the substellar limit. By using the location of the stars on
color-magnitude and color-color diagrams, we have selected candidate members of
this cluster. We have merged our sample with previously published data and
obtained a color-magnitude diagram for the complete stellar population of the
cluster, covering the spectral range early B - mid M. The Mass Function
increases monotonically, when plotted in a log-log form, until it reaches ~0.8
M_\odot (\alpha=2.59). It remains shallower for less massive stars (\alpha=0.81
for 0.8-0.2 M_\odot), whereas a decrease ins observed for stars close to the
substellar regime. The total mass of the cluster is ~1600 M_\odot in the area
covered by this study.Comment: Accepted ApJ (Jan 10, 2001 issue
The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)
We continue our study of rich Galactic clusters by presenting deep CCD
observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found
to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC
2323 contains approximately 2100 stars brighter than our photometric limit of V
= 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The
color-magnitude diagrams for the clusters exhibit clear main sequences
stretching over 14 magnitudes in the V, B-V plane. Comparing these long main
sequences with those of earlier clusters in the survey, as well as with the
Hyades, has allowed for accurate distances to be established for each cluster
(dist. of NGC 2168 = 912 +/- 70/65 pc, dist. of NGC 2323 = 1000 +/- 81/75 pc).
Analysis of the luminosity and mass functions suggest that despite their young
ages, both clusters are somewhat dynamically relaxed exhibiting signs of
mass-segregation. This is especially interesting in the case of NGC 2323, which
has an age of only 1.3 times the dynamical relaxation time. The present
photometry is also deep enough to detect all of the white dwarfs in both
clusters. We discuss some interesting candidates which may be the remnants of
quite massive (M > 5 Mo) progenitor stars. The white dwarf cooling age of NGC
2168 is found to be in good agreement with the main-sequence turn-off age.
These objects are potentially very important for setting constraints on the
white dwarf initial-final mass relationship and upper mass limit for white
dwarf production.Comment: 34 pages, including 12 diagrams and 5 tables. Accepted for
publication in AJ. Minor typos correcte
Using dissolved H<sub>2</sub>O in rhyolitic glasses to estimate palaeo-ice thickness during a subglacial eruption at BlĂĄhnĂșkur(Torfajökull, Iceland)
The last decade has seen the refinement of a technique for reconstructing palaeo-ice thicknesses based on using the retained H2O and CO2 content in glassy eruptive deposits to infer quenching pressures and therefore ice thicknesses. The method is here applied to BlĂĄhnĂșkur, a subglacially erupted rhyolitic edifice in Iceland. A decrease in water content from ~0.7 wt.% at the base to ~0.3 wt.% at the top of the edifice suggests that the ice was 400 m thick at the time of the eruption. As BlĂĄhnĂșkur rises 350 m above the surrounding terrain, this implies that the eruption occurred entirely within ice, which corroborates evidence obtained from earlier lithofacies studies. This paper presents the largest data set (40 samples) so far obtained for the retained volatile contents of deposits from a subglacial eruption. An important consequence is that it enables subtle but significant variations in water content to become evident. In particular, there are anomalous samples which are either water-rich (up to 1 wt.%) or water-poor (~0.2 wt.%), with the former being interpreted as forming intrusively within hyaloclastite and the latter representing batches of magma that were volatile-poor prior to eruption. The large data set also provides further insights into the strengths and weaknesses of using volatiles to infer palaeo-ice thicknesses and highlights many of the uncertainties involved. By using examples from BlĂĄhnĂșkur, the quantitative use of this technique is evaluated. However, the relative pressure conditions which have shed light on BlĂĄhnĂșkurâs eruption mechanisms and syn-eruptive glacier response show that, despite uncertainties in absolute values, the volatile approach can provide useful insight into the mechanisms of subglacial rhyolitic eruptions, which have never been observed
Seasonal temperatures in West Antarctica during the Holocene
The recovery of long-term climate proxy records with seasonal resolution is rare because of natural smoothing processes, discontinuities and limitations in measurement resolution. Yet insolation forcing, a primary driver of multimillennial-scale climate change, acts through seasonal variations with direct impacts on seasonal climate1. Whether the sensitivity of seasonal climate to insolation matches theoretical predictions has not been assessed over long timescales. Here, we analyse a continuous record of water-isotope ratios from the West Antarctic Ice Sheet Divide ice core to reveal summer and winter temperature changes through the last 11,000âyears. Summer temperatures in West Antarctica increased through the early-to-mid-Holocene, reached a peak 4,100âyears ago and then decreased to the present. Climate model simulations show that these variations primarily reflect changes in maximum summer insolation, confirming the general connection between seasonal insolation and warming and demonstrating the importance of insolation intensity rather than seasonally integrated insolation or season duration2,3. Winter temperatures varied less overall, consistent with predictions from insolation forcing, but also fluctuated in the early Holocene, probably owing to changes in meridional heat transport. The magnitudes of summer and winter temperature changes constrain the lowering of the West Antarctic Ice Sheet surface since the early Holocene to less than 162âm and probably less than 58âm, consistent with geological constraints elsewhere in West Antarctica4-7
Southern Hemisphere climate variability forced by Northern Hemisphere ice-sheet topography
The presence of large Northern Hemisphere ice sheets and reduced greenhouse gas concentrations during the Last Glacial Maximum fundamentally altered global oceanâatmosphere climate dynamics1. Model simulations and palaeoclimate records suggest that glacial boundary conditions affected the El NiñoâSouthern Oscillation2,3, a dominant source of short-term global climate variability. Yet little is known about changes in short-term climate variability at mid- to high latitudes. Here we use a high-resolution water isotope record from West Antarctica to demonstrate that interannual to decadal climate variability at high southern latitudes was almost twice as large at the Last Glacial Maximum as during the ensuing Holocene epoch (the past 11,700 years). Climate model simulations indicate that this increased variability reflects an increase in the teleconnection strength between the tropical Pacific and West Antarctica, owing to a shift in the mean location of tropical convection. This shift, in turn, can be attributed to the influence of topography and albedo of the North American ice sheets on atmospheric circulation. As the planet deglaciated, the largest and most abrupt decline in teleconnection strength occurred between approximately 16,000 years and 15,000 years ago, followed by a slower decline into the early Holocene
Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming
Ice flow along land-terminating margins of the Greenland Ice Sheet (GIS) varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet. Such inputs lubricate the ice-bed interface, transiently speeding up the flow of ice. Greater melting results in faster ice motion during summer, but slower motion over the subsequent winter, owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice-sheet bed that have a high basal water pressure. However, the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we show that annual ice motion across an 8,000-km2 land-terminating region of the west GIS margin, extending to 1,100 m above sea level, was 12 slower in 2007-14 compared with 1985-94, despite a 50 increase in surface meltwater production. Our findings suggest that, over these three decades, hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion (not a speed-up, as previously postulated). Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the GIS. Our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought. Ă© 2015 Macmillan Publishers Limited. All rights reserved
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Precise interpolar phasing of abrupt climate change during the last ice age
The last glacial period exhibited abrupt DansgaardâOeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archivesÂč. Ice cores show that Antarctica cooled during the warm phases of the Greenland DansgaardâOeschger cycle and vice versa[superscript 2,3], suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw[superscript 4â6]. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events[superscript 7â9]. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision[superscript 2,3,10]. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2Ï) for DansgaardâOeschger events, including the BĂžlling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of DansgaardâOeschger dynamics
Data Descriptor: A global multiproxy database for temperature reconstructions of the Common Era
Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850-2014. Global temperature composites show a remarkable degree of coherence between high-and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.(TABLE)Since the pioneering work of D'Arrigo and Jacoby1-3, as well as Mann et al. 4,5, temperature reconstructions of the Common Era have become a key component of climate assessments6-9. Such reconstructions depend strongly on the composition of the underlying network of climate proxies10, and it is therefore critical for the climate community to have access to a community-vetted, quality-controlled database of temperature-sensitive records stored in a self-describing format. The Past Global Changes (PAGES) 2k consortium, a self-organized, international group of experts, recently assembled such a database, and used it to reconstruct surface temperature over continental-scale regions11 (hereafter, ` PAGES2k-2013').This data descriptor presents version 2.0.0 of the PAGES2k proxy temperature database (Data Citation 1). It augments the PAGES2k-2013 collection of terrestrial records with marine records assembled by the Ocean2k working group at centennial12 and annual13 time scales. In addition to these previously published data compilations, this version includes substantially more records, extensive new metadata, and validation. Furthermore, the selection criteria for records included in this version are applied more uniformly and transparently across regions, resulting in a more cohesive data product.This data descriptor describes the contents of the database, the criteria for inclusion, and quantifies the relation of each record with instrumental temperature. In addition, the paleotemperature time series are summarized as composites to highlight the most salient decadal-to centennial-scale behaviour of the dataset and check mutual consistency between paleoclimate archives. We provide extensive Matlab code to probe the database-processing, filtering and aggregating it in various ways to investigate temperature variability over the Common Era. The unique approach to data stewardship and code-sharing employed here is designed to enable an unprecedented scale of investigation of the temperature history of the Common Era, by the scientific community and citizen-scientists alike
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