70 research outputs found
Testing circumstellar disk lifetimes in young embedded clusters associated with the Vela Molecular Ridge
Context. The Vela Molecular Ridge hosts a number of young embedded star
clusters in the same evolutionary stage. Aims. The main aim of the present work
is testing whether the fraction of members with a circumstellar disk in a
sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent
with relations derived for larger samples of star clusters with an age spread.
Besides, we want to constrain the age of the young embedded star clusters
associated with cloud D. Methods. We carried out L (3.78 microns) photometry on
images of six young embedded star clusters associated with cloud D of the Vela
Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with
the available HKs photometry. The 6 clusters are roughly of the same size and
appear to be in the same evolutionary stage. The fraction of stars with a
circumstellar disk was measured in each cluster by counting the fraction of
sources displaying a NIR excess in colour-colour (HKsL) diagrams. Results. The
L photometry allowed us to identify the NIR counterparts of the IRAS sources
associated with the clusters. The fraction of stars with a circumstellar disk
appears to be constant within errors for the 6 clusters. There is a hint that
this is lower for the most massive stars. The age of the clusters is
constrained to ~1-2 Myr. Conclusions. The fraction of stars with a
circumstellar disk in the observed sample is consistent with the relations
derived from larger samples of star clusters and with other age estimates for
cloud D. The fraction may be lower for the most massive stars. Our results
agree with a scenario where all intermediate and low-mass stars form with a
disk, whose lifetime is shorter for higher mass stars.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Protein conformation and molecular order probed by second-harmonic-generation microscopy
Second-harmonic-generation (SHG) microscopy has emerged as a powerful tool to image unstained living tissues and probe their molecular and supramolecular organization. In this article, we review the physical basis of SHG, highlighting how coherent summation of second-harmonic response leads to the sensitivity of polarized SHG to the three-dimensional distribution of emitters within the focal volume. Based on the physical description of the process, we examine experimental applications for probing the molecular organization within a tissue and its alterations in response to different biomedically relevant conditions. We also describe the approach for obtaining information on molecular conformation based on SHG polarization anisotropy measurements and its application to the study of myosin conformation in different physiological states of muscle. The capability of coupling the advantages of nonlinear microscopy (micrometer-scale resolution in deep tissue) with tools for probing molecular structure in vivo renders SHG microscopy an extremely powerful tool for the advancement of biomedical optics, with particular regard to novel technologies for molecular diagnostic in vivo. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE)
Delayed Photoionization Feedback in a Super Star Cluster in SBS0335-052E
SBS0335-052 is a well studied Blue Compact Dwarf galaxy with one of the
lowest metallicities of any known galaxy. It also contains 6 previously
identified Super Star Clusters. We combine archival HST NICMOS images in the Pa
alpha line and the 1.6 micron continuum of the eastern component, SBS0335-052E,
with other space and ground based data to perform a multi-wavelength analysis
of the super star clusters. We concentrate on the southern most clusters,
designated S1 and S2, which appear to be the youngest clusters and are the
strongest emitters of Pa alpha, radio, and x-ray flux. Our analysis leads to a
possible model for S1 and perhaps S2 as a cluster of very young, massive stars
with strong stellar winds. The wind density can be high enough to absorb the
majority of ionizing photons within less than 1000 AU of the stars, creating
very compact HII regions that emit optically thick radiation at radio
wavelengths. These winds would then effectively quench the photoionizing flux
very close to the stars. This can delay the onset of negative feedback by
photoionization and photodissociation on star formation in the clusters. This
is significant since SBS0335-052E resembles the conditions that were probably
common for high redshift star formation in galaxies near the epoch of
reionization.Comment: Accepted for publication in the Astrophysical Journa
Lac repressor hinge flexibility and DNA looping: single molecule kinetics by tethered particle motion
The tethered particle motion (TPM) allows the direct detection of activity of a variety of biomolecules at the single molecule level. First pioneered for RNA polymerase, it has recently been applied also to other enzymes. In this work we employ TPM for a systematic investigation of the kinetics of DNA looping by wild-type Lac repressor (wt-LacI) and by hinge mutants Q60G and Q60 + 1. We implement a novel method for TPM data analysis to reliably measure the kinetics of loop formation and disruption and to quantify the effects of the protein hinge flexibility and of DNA loop strain on such kinetics. We demonstrate that the flexibility of the protein hinge has a profound effect on the lifetime of the looped state. Our measurements also show that the DNA bending energy plays a minor role on loop disruption kinetics, while a strong effect is seen on the kinetics of loop formation. These observations substantiate the growing number of theoretical studies aimed at characterizing the effects of DNA flexibility, tension and torsion on the kinetics of protein binding and dissociation, strengthening the idea that these mechanical factors in vivo may play an important role in the modulation of gene expression regulation
The MUSE view of He 2-10: no AGN ionization but a sparkling starburst
We study the physical and dynamical properties of the ionized gas in the
prototypical HII galaxy Henize 2-10 using MUSE integral field spectroscopy. The
large scale dynamics is dominated by extended outflowing bubbles, probably the
results of massive gas ejection from the central star forming regions. We
derive a mass outflow rate dMout/dt~0.30 Msun/yr, corresponding to mass loading
factor eta~0.4, in range with similar measurements in local LIRGs. Such a
massive outflow has a total kinetic energy that is sustainable by the stellar
winds and Supernova Remnants expected in the galaxy. We use classical emission
line diagnostic to study the dust extinction, electron density and ionization
conditions all across the galaxy, confirming the extreme nature of the highly
star forming knots in the core of the galaxy, which show high density and high
ionization parameter. We measure the gas phase metallicity in the galaxy taking
into account the strong variation of the ionization parameter, finding that the
external parts of the galaxy have abundances as low as 12 + log(O/H)~8.3, while
the central star forming knots are highly enriched with super solar
metallicity. We find no sign of AGN ionization in the galaxy, despite the
recent claim of the presence of a super massive active Black Hole in the core
of He~2-10. We therefore reanalyze the X-ray data that were used to propose the
presence of the AGN, but we conclude that the observed X-ray emission can be
better explained with sources of a different nature, such as a Supernova
Remnant.Comment: 11 pages, 8 Figures, Accepted for publication in A&
Bessel beam illumination reduces random and systematic errors in quantitative functional studies using light-sheet microscopy
Light-sheet microscopy (LSM), in combination with intrinsically transparent zebrafish larvae, is a choice method to observe brain function with high frame rates at cellular resolution. Inherently to LSM, however, residual opaque objects cause stripe artifacts, which obscure features of interest and, during functional imaging, modulate fluorescence variations related to neuronal activity. Here, we report how Bessel beams reduce streaking artifacts and produce high-fidelity quantitative data demonstrating a fivefold increase in sensitivity to calcium transients and a 20 fold increase in accuracy in the detection of activity correlations in functional imaging. Furthermore, using principal component analysis, we show that measurements obtained with Bessel beams are clean enough to reveal in one-shot experiments correlations that can not be averaged over trials after stimuli as is the case when studying spontaneous activity. Our results not only demonstrate the contamination of data by systematic and random errors through conventional Gaussian illumination and but,furthermore, quantify the increase in fidelity of such data when using Bessel beams
Super Star Clusters in SBS0335-052E
As one of the lowest metallicity star forming galaxies, with a nucleus of
several super star clusters, SBS0335-052E is the subject of substantial current
study. We present new insights on this galaxy based on new and archival high
spatial resolution NICMOS and ACS images. We provide new measurements and
limits on the size of several of the SSCs. The images have sufficient
resolution to divide the star formation into compact regions and newly
discovered extended regions, indicating a bi-modal form of star formation. The
star formation regions are dated via the equivalent width of the Pa alpha
emission and we find that two of the extended regions of star formation are
less than 10 million years old. Our previous finding that stellar winds confine
the photo-ionizing flux to small regions around individual stars is consistent
with the new observations. This may allow planet formation in what would
traditionally be considered a harsh environment and has implications for the
number of planets around globular cluster stars. In addition the images
pinpoint the regions of H2 emission as located in, but not at the center of the
two star forming super star clusters, S1 and S2.Comment: Accepted by the Astrophysical Journa
Second-harmonic generation sensitivity to transmembrane potential in normal and tumor cells.
Second-harmonic generation (SHG) is emerging as a powerful tool for the optical measurement of transmembrane potential in live cells with high sensitivity and temporal resolution. Using a patch clamp, we characterize the sensitivity of the SHG signal to transmembrane potential for the RH 237 dye in various normal and tumor cell types. SHG sensitivity shows a significant dependence on the type of cell, ranging from 10 to 17% per 100 mV. Furthermore, in the samples studied, tumor cell lines display a higher sensitivity compared to normal cells. In particular, the SHG sensitivity increases in the cell line Balb/c3T3 by the transformation induced with SV40 infection of the cells. We also demonstrate that fluorescent labeling of the membrane with RH 237 at the concentration used for SHG measurements does not induce any measurable alteration in the electrophysiological properties of the cells investigated. Therefore, SHG is suitable for the investigation of outstanding questions in electrophysiology and neurobiology
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