2,161 research outputs found
Specific and non specific hybridization of oligonucleotide probes on microarrays
Gene expression analysis by means of microarrays is based on the sequence
specific binding of mRNA to DNA oligonucleotide probes and its measurement
using fluorescent labels. The binding of RNA fragments involving other
sequences than the intended target is problematic because it adds a "chemical
background" to the signal, which is not related to the expression degree of the
target gene. The paper presents a molecular signature of specific and non
specific hybridization with potential consequences for gene expression
analysis. We analyzed the signal intensities of perfect match (PM) and mismatch
(MM) probes of GeneChip microarrays to specify the effect of specific and non
specific hybridization. We found that these events give rise to different
relations between the PM and MM intensities as function of the middle base of
the PMs, namely a triplet- (C>G=T>A>0) and a duplet-like (C=T>0>G=A) pattern of
the PM-MM log-intensity difference upon binding of specific and non specific
RNA fragments, respectively. The systematic behaviour of the intensity
difference can be rationalized on the level of base pairings of DNA/RNA
oligonucleotide duplexes in the middle of the probe sequence. Non-specific
binding is characterized by the reversal of the central Watson Crick (WC)
pairing for each PM/MM probe pair, whereas specific binding refers to the
combination of a WC and a self complementary (SC) pairing in PM and MM probes,
respectively. The intensity of complementary MM introduces a systematic source
of variation which decreases the precision of expression measures based on the
MM intensities
Sequentially Triggered Star Formation in OB Associations
We discuss observational evidence for sequential and triggered star formation
in OB associations. We first review the star formation process in the
Scorpius-Centaurus OB association, the nearest OB association to the Sun, where
several recent extensive studies have allowed us to reconstruct the star
formation history in a rather detailed way. We then compare the observational
results with those obtained for other OB associations and with recent models of
rapid cloud and star formation in the turbulent interstellar medium. We
conclude that the formation of whole OB subgroups (each consisting of several
thousand stars) requires large-scale triggering mechanisms such as shocks from
expanding wind and supernova driven superbubbles surrounding older subgroups.
Other triggering mechanisms, like radiatively driven implosion of globules,
also operate, but seem to be secondary processes, forming only small stellar
groups rather than whole OB subgroups with thousands of stars.Comment: Invited talk at the IAU Symposium 237: "Triggered Star Formation in a
Turbulent ISM", Prague, Czech Republic, August 200
X-ray view of IC348 in the light of an updated cluster census
We study the properties of the coronae of the low-mass stars in the young
(~2-3Myr), nearby (~310pc) open cluster IC348 combining X-ray and
optical/infrared data. The four existing Chandra observations of IC348 are
merged, thus providing a deeper and spatially more complete X-ray view than
previous X-ray studies of the cluster. We have compiled a comprehensive catalog
of IC348 members taking into account recent updates to the cluster census. Our
data collection comprises fundamental stellar parameters, infrared excess
indicating the presence of disks, Halpha emission as a tracer of chromospheric
emission or accretion and mass accretion rates. We have detected 290 X-ray
sources in four merged Chandra exposures, of which 187 are associated with
known cluster members. Only four of the X-ray sources are brown dwarfs
(spectral type M6 and later). The detection rate is highest for diskless Class
III stars and increases with stellar mass. This may be explained with higher
X-ray luminosities for higher mass and later evolutionary stage that is evident
in the X-ray luminosity functions. In particular, we find that for the lowest
examined masses (0.1-0.25 Msun) there is a difference between the X-ray
luminosity functions of accreting and non-accreting stars (classified on the
basis of their Halpha emission strength) as well as those of disk-bearing and
diskless stars (classified on the basis of the slope of the spectral energy
distribution). These differences disappear for higher masses. This is related
to our finding that the L_x/L_bol ratio is non-constant across the
mass/luminosity sequence of IC348 with a decrease towards lower luminosity
stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster
suggests that the decline of L_x/L_ bol for young stars at the low-mass end of
the stellar sequence is likely universal.Comment: Accepted for publication in Astronomy & Astrophysic
Age spreads in clusters and associations: the lithium test
We report the evidence that several low-mass stars (<~0.4 Msun) of the Orion
and Upper Scorpius clusters have lithium abundances well below the interstellar
value. Due to time-dependent depletion, our result implies stellar ages greater
than ~5 Myr, suggesting that star formation has been proceeding for a long time
in these systems.Comment: to appear in IMF@50: The Initial Mass Function 50 years later, eds.
E. Corbelli et al. (Kluwer Acad. Press), 2004, in pres
The Clump Mass Function of the Dense Clouds in the Carina Nebula Complex
We want to characterize the properties of the cold dust clumps in the Carina
Nebula Complex (CNC), which shows a very high level of massive star feedback.
We derive the Clump Mass Function (ClMF), explore the reliability of different
clump extraction algorithms, and investigate the influence of the temperatures
within the clouds on the resulting shape of the ClMF.
We analyze a 1.25x1.25 deg^2 wide-field sub-mm map obtained with LABOCA
(APEX), which provides the first spatially complete survey of the clouds in the
CNC. We use the three clump-finding algorithms CLUMPFIND (CF), GAUSSCLUMPS (GC)
and SExtractor (SE) to identify individual clumps and determine their total
fluxes. In addition to assuming a common `typical' temperature for all clouds,
we also employ an empirical relation between cloud column densities and
temperature to determine an estimate of the individual clump temperatures, and
use this to determine individual clump masses.
While the ClMF based on the CF extraction is very well described by a
power-law, the ClMFs based on GC and SE are better represented by a log-normal
distribution. We also find that the use of individual clump temperatures leads
to a shallower ClMF slope than the assumption of a common temperature (e.g. 20
K) of all clumps.
The power-law of dN/dM \propto M^-1.95 we find for the CF sample is in good
agreement with ClMF slopes found in previous studies of other regions. The
dependence of the ClMF shape (power-law vs. log-normal distribution) on the
employed extraction method suggests that observational determinations of the
ClMF shape yields only very limited information about the true structure of the
cloud. Interpretations of log-normal ClMF shape as a signature of turbulent
pre-stellar clouds vs. power-law ClMFs as a signature of star-forming clouds
may be taken with caution for a single extraction algorithm without additional
information.Comment: 8 pages, 7 figures, accepted by A&
Direct Imaging and Spectroscopy of a Planetary Mass Candidate Companion to a Young Solar Analog
We present Gemini near-infrared adaptive optics imaging and spectroscopy of a
planetary mass candidate companion to 1RXS J160929.1-210524, a roughly
solar-mass member of the 5 Myr-old Upper Scorpius association. The object,
separated by 2.22" or 330 AU at ~150 pc, has infrared colors and spectra
suggesting a temperature of 1800(-100/+200) K, and spectral type of L4(-2/+1).
The H- and K-band spectra provide clear evidence of low surface gravity, and
thus youth. Based on the widely used DUSTY models, we infer a mass of
8(-2/+4)Mjupiter. If gravitationally bound, this would be the lowest mass
companion imaged around a normal star thus far, and its existence at such a
large separation would pose a serious challenge to theories of star and planet
formation.Comment: Revised accepted version, ApJL, in pres
The onset of X-ray emission in young stellar objects: a Chandra observation of the Serpens star-forming region
AIMS: To study the properties of X-ray emissions from young stellar objects
(YSOs), through their evolution from Class I to Class III and determine whether
Class 0 protostars emit in X-rays. METHODS: A deep Chandra X-ray observation of
the Serpens star-forming region was obtained. The Serpens Cloud Core is ideally
suited for this type of investigation, being populated by a dense and extremely
young cluster whose members are found in all different evolutionary stages,
including six well studied Class 0 sources. RESULTS: None of the six Class 0
protostars is detected in our observations, excluding the presence of sources
with X-ray luminosities > 0.4 10^30 erg/s (for column densities of the order of
4 10^{23} cm^-2, or A_V ~ 200). A total of 85 X-ray sources are detected and
the light curves and spectra of 35 YSOs are derived. There is a clear trend of
decreasing absorbing column densities as one moves from Class I to Class III
sources, and, possibly, evidence of decreasing plasma temperatures, too. We
observe a strong, long-duration, flare from a Class II low-mass star, for which
we derive a flaring loop length of the order of 20 stellar radii. We interpret
the flaring event as originating from a magnetic flux tube connecting the star
to its circumstellar disk. The presence of such a disk is supported by the
detection, in the spectrum of this star, of 6.4 keV Fe fluorescent emission.Comment: Accepted for publication in A&
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