Statistical Confirmation of a Stellar Upper Mass Limit

We derive the expectation value for the maximum stellar mass (m_max) in an ensemble of N stars, as a function of the IMF upper-mass cutoff (m_up) and N. We statistically demonstrate that the upper IMF of the local massive star census observed thus far in the Milky Way and Magellanic Clouds clearly exhibits a universal upper mass cutoff around 120 - 200 M_sun for a Salpeter IMF, although the result is more ambiguous for a steeper IMF.Comment: PDF, 5 pages, 4 figures. Accepted to the Astrophysical Journal Letter

The ionising cluster of 30 Doradus.IV. Stellar kinematics

On the basis of multislit spectroscopy of 180 stars in the ionising cluster of 30 Doradus we present reliable radial velocities for 55 stars. We calculate a radial velocity dispersion of ~35 km/s for the cluster and we analyse the possible influence of spectroscopic binaries in this rather large velocity dispersion. We use numerical simulations to show that the observations are consistent with the hypothesis that all the stars in the cluster are binaries, and the total mass of the cluster is ~5E+5 solar masses. A simple test shows only marginal evidence for dynamical mass segregation which if present is most likely not due to dynamical relaxation.Comment: accepted for publication in Astronomy and Astrophysic

Comparıng pulse pressure varıatıon and pleth varıabılıty ındex in the semı-recumbent and trendelenburg posıtıon ın crıtıcally ıll septıc patıents

Introduction. Dynamic tests for predicting fluid responsiveness have generated increased interest in recent years. One of these tests, pulse pressure variation (PPV), is a parameter calculated from respiratory variations of pulse pressure. Another test, pleth variability index (PVI), is based on respiratory variations of the perfusion index and can be measured non-invasively by pulse oximeter. Previous studies have shown that both tests are valuable in determining fluid responsiveness. Methods. In this observational prospective study, our aim was to compare the PVI and PPV in order to identify a convenient tool for determining fluid responsiveness. Our study was performed in a surgical and reanimation intensive care unit. We enrolled one hundred mechanically ventilated adult patients diagnosed with sepsis. Exclusion criteria included brain death, spontaneous breathing, cardiac arrhythmia, and impaired peripheral circulation. We measured the PPV by arterial monitorization and the PVI by using Masimo Radical 7 in the 45° semi-recumbent position (SP) and then 15° Trendelenbug position (TP). We performed correlation and ROC analysis using a >13% fluid responsiveness cut-off value for the PPV and >14% for the PVI. Results. Between the SP and the TP, we did not observe significant decreases in PPV (from 14.17 ± 10.57 to 12.66 ± 9.64; p > 0.05), while we did observe significant decreases in PVI (from 21.91 ± 13.99 to 20.46 ± 14.12; p < 0.05). The PPV fluid responsiveness cut-off value in the SP and TP was 20% (78.95% sensitivity, 77.05% specificity) and 18% (76.67% sensitivity, 72.46% specificity), respectively. The PVI fluid responsiveness cut-off value in the SP and TP was 20% (80.49% sensitivity, 81.03% specificity) and 16% (81.25% sensitivity, 62.69% specificity), respectively. The area under the ROC of the PPV and PVI was 0.843 and 0.858 in the SP, respectively, and 0.760 and 0.747 in the TP, respectively. The PPV and PVI were correlated in the SP (r = 0.578; p = 0.001) and the TP (r = 0.517; p = 0.001). Conclusions. Our results showed that the PPV and PVI were correlated independent of position change in sepsis patients. Both tests appear to be equivalently reliable. However, the ability of the PPV and PVI to predict fluid responsiveness decreased in the TP in our study

An Optical and X-ray Examination of Two Radio Supernova Remnant Candidates in 30 Doradus

The giant HII region 30 Doradus is known for its violent internal motions and bright diffuse X-ray emission, suggesting the existence of supernova remnants (SNRs), but no nonthermal radio emission has been detected. Recently, Lazendic et al. compared the H-alpha/H-beta and radio/H-alpha ratios and suggested two small radio sources to be nonthermal and thus SNR candidates; however, no optical or X-ray counterparts were detected. We have used high-resolution optical images and high-dispersion spectra to examine the morphological, spectral, and kinematic properties of these two SNR candidates, and still find no optical evidence supporting their identification as SNRs. We have also determined the X-ray luminosities of these SNR candidates, and find them 1-3 orders of magnitude lower than those commonly seen in young SNRs. High extinction can obscure optical and X-ray signatures of an SNR, but would prohibit the use of a high radio/H-alpha ratio to identify nonthermal radio emission. We suggest that the SNR candidate MCRX J053831.8-690620 is associated with a young star forming region; while the radio emission originates from the obscured star forming region, the observed optical emission is dominated by the foreground. We suggest that the SNR candidate MCRX J053838.8-690730 is associated with a dust/molecular cloud, which obscures some optical emission but not the radio emission.Comment: 13 pages, 2 figures, accepted for publication in the ApJ, Nov 10, 200

On random graphs and the statistical mechanics of granular matter

The dynamics of spins on a random graph with ferromagnetic three-spin interactions is used to model the compaction of granular matter under a series of taps. Taps are modelled as the random flipping of a small fraction of the spins followed by a quench at zero temperature. We find that the density approached during a logarithmically slow compaction - the random-close-packing density - corresponds to a dynamical phase transition. We discuss the the role of cascades of successive spin-flips in this model and link them with density-noise power fluctuations observed in recent experiments.Comment: minor changes, to appear in EP

Common and unique transcriptional responses to dietary restriction and loss of insulin receptor substrate 1 (IRS1) in mice

Dietary restriction (DR) is the most widely studied non-genetic intervention capable of extending lifespan across multiple taxa. Modulation of genes, primarily within the insulin/insulin-like growth factor signalling (IIS) and the mechanistic target of rapamycin (mTOR) signalling pathways also act to extend lifespan in model organisms. For example, mice lacking insulin receptor substrate-1 (IRS1) are long-lived and protected against several age-associated pathologies. However, it remains unclear how these particular interventions act mechanistically to produce their beneficial effects. Here, we investigated transcriptional responses in wild-type and IRS1 null mice fed an ad libitum diet (WTAL and KOAL) or fed a 30% DR diet (WTDR or KODR). Using an RNAseq approach we noted a high correlation coefficient of differentially expressed genes existed within the same tissue across WTDR and KOAL mice and many metabolic features were shared between these mice. Overall, we report that significant overlap exists in the tissue-specific transcriptional response between long-lived DR mice and IRS1 null mice. However, there was evidence of disconnect between transcriptional signatures and certain phenotypic measures between KOAL and KODR, in that additive effects on body mass were observed but at the transcriptional level DR induced a unique set of genes in these already long-lived mice

The Effect of Star Formation History on the Inferred Initial Stellar Mass Function

Peaks and lulls in the star formation rate (SFR) over the history of the Galaxy produce plateaux and declines in the present day mass function (PDMF) where the main-sequence lifetime overlaps the age and duration of the SFR variation. These PDMF features can be misinterpreted as the form of the intrinsic stellar initial mass function (IMF) if the star formation rate is assumed to be constant or slowly varying with time. This effect applies to all regions that have formed stars for longer than the age of the most massive stars, including OB associations, star complexes, and especially galactic field stars. Related problems may apply to embedded clusters. Evidence is summarized for temporal SFR variations from parsec scales to entire galaxies, all of which should contribute to inferred IMF distortions. We give examples of various star formation histories to demonstrate the types of false IMF structures that might be seen. These include short-duration bursts, stochastic histories with log-normal amplitude distributions, and oscillating histories with various periods and phases. The inferred IMF should appear steeper than the intrinsic IMF over mass ranges where the stellar lifetimes correspond to times of decreasing SFRs; shallow portions of the inferred IMF correspond to times of increasing SFRs. If field regions are populated by dispersed clusters and defined by their low current SFRs, then they should have steeper inferred IMFs than the clusters. The SFRs required to give the steep field IMFs in the LMC and SMC are determined. Structure observed in several determinations of the Milky Way field star IMF can be accounted for by a stochastic and bursty star formation history.Comment: accepted by ApJ for 1 Jan 2006, Vol 636, 12 pages + 6 figure