48 research outputs found
VLBA determination of the distance to nearby star-forming regions I. The distance to T Tauri with 0.4% accuracy
In this article, we present the results of a series of twelve 3.6-cm radio
continuum observations of T Tau Sb, one of the companions of the famous young
stellar object T Tauri. The data were collected roughly every two months
between September 2003 and July 2005 with the Very Long Baseline Array (VLBA).
Thanks to the remarkably accurate astrometry delivered by the VLBA, the
absolute position of T Tau Sb could be measured with a precision typically
better than about 100 micro-arcseconds at each of the twelve observed epochs.
The trajectory of T Tau Sb on the plane of the sky could, therefore, be traced
very precisely, and modeled as the superposition of the trigonometric parallax
of the source and an accelerated proper motion. The best fit yields a distance
to T Tau Sb of 147.6 +/- 0.6 pc. The observed positions of T Tau Sb are in good
agreement with recent infrared measurements, but seem to favor a somewhat
longer orbital period than that recently reported by Duchene et al. (2006) for
the T Tau Sa/T Tau Sb system.Comment: 24 pages, 3 pages, AASTEX format, accepted for publication in Ap
HST/WFPC2 Study of the Trapezium Cluster: the Influence of Circumstellar Disks on the Initial Mass Function
We have performed the first measures of mass accretion rates in the core of
the Orion Nebula Cluster. Four adjacent fields centered on the Trapezium stars
have been imaged in the U- and B-bands using the Wide Field Planetary Camera 2
on board the Hubble Space Telescope. In this paper we focus our attention on a
group of 40 stars with known spectral types and complete UBVI WFPC2 photometry.
Approximately three quarters of the sources show excess luminosity in the
U-band, that we attribute to mass accretion. The known correlation between the
U-band excess and the total accretion luminosity allows us to estimate the
accretion rates. Overall, mass accretion rates appear lower than those measured
by other authors in the Orion flanking fields or in Taurus-Auriga. Mass
accretion rates remain low even in the vicinity of the birth line of Palla &
Stahler, suggesting that in the core of the Trapezium cluster disk accretion
has been recently depressed by an external mechanism. We suggest that the UV
radiation generated by the Trapezium OB stars, responsible for the disk
evaporation, may also cause the drop of the mass accretion rate. In this
scenario, low-mass stars may terminate their pre-main sequence evolution with
masses lower than those they would have reached if disk accretion could have
proceeded undisturbed until the final disk consumption. In OB associations the
low-mass end of the Initial Mass Function may therefore be affected by the
rapid evolution of the most massive cluster's stars, causing a surplus
"accretion aborted" very low-mass stars and brown dwarfs, and a deficit of
intermediate mass stars. This trend is in agreement with recent observations of
the IMF in the Trapezium cluster.Comment: 24 pages, 12 figures, 4 table
A rotational and variability study of a large sample of PMS stars in NGC 2264
We present the results of an extensive search for periodic and irregular
variable pre-main sequence (PMS) stars in the young (2-4 Myr) open cluster NGC
2664, based on photometric monitoring using the Wide Field Imager (WFI) on the
2.2m telescope on La Silla (Chile). In total, about 10600 stars with I_c
magnitudes between 9.8mag and 21mag have been monitored in our 34x33 arcmin
field. Time series data were obtained in the I_c band in 44 nights between Dec.
2000 and March 2001; altogether we obtained 88 data points per star. Using two
different time series analysis techniques (Scargle and CLEAN) we found 543
periodically variable stars with periods between 0.2 days and 15 days. Also,
484 irregular variable stars were identified using a chi-squared-test. In
addition we have carried out nearly simultaneous observations in V, R_c and a
narrow-band Halpha filter. The photometric data enable us to reject background
and foreground stars from our sample of variable stars according to their
location in the I_c vs. (R_c-I_c) colour-magnitude and (R_c-Halpha) vs.
(R_c-I_c) colour-colour diagram. We could in the end identify 405 periodically
variable and 184 irregular variable PMS stars as cluster members using these
two different tests. In addition 35 PMS stars for which no significant
variablilty were detected could be identified as members using an Halpha
emission index criterion. This yields to a total of 624 PMS stars in NGC 2264,
of which only 182 were known before. Most of the newly found PMS stars are
fainter than I_c = 16 and of late spectral type (>M2). (abridged)Comment: To appear in Astronomy & Astrophysics. Several figures missing due to
size constraints. Full paper (25 pages) available from
http://www.mpia-hd.mpg.de/homes/calj/ngc2264_p1.htm
Current laboratory and clinical practices in reporting and interpreting anti?nuclear antibody indirect immunofluorescence (ANA IIF) patterns: results of an international survey
Background:
The International Consensus on Antinuclear Antibody (ANA) Patterns (ICAP) has recently proposed nomenclature in order to harmonize ANA indirect immunofluorescence (IIF) pattern reporting. ICAP distinguishes competent-level from expert-level patterns. A survey was organized to evaluate reporting, familiarity, and considered clinical value of ANA IIF patterns.
Methods:
Two surveys were distributed by European Autoimmunity Standardization Initiative (EASI) working groups, the International Consensus on ANA Patterns (ICAP) and UK NEQAS to laboratory professionals and clinicians.
Results:
438 laboratory professionals and 248 clinicians from 67 countries responded. Except for dense fine speckled (DFS), the nuclear competent patterns were reported by>85% of the laboratories. Except for rods and rings, the cytoplasmic competent patterns were reported by>72% of laboratories.
Cytoplasmic IIF staining was considered ANA positive by 55% of clinicians and 62% of laboratory professionals, with geographical and expertise-related differences.
Quantification of fluorescence intensity was considered clinically relevant for nuclear patterns, but less so for cytoplasmic and mitotic patterns. Combining IIF with specific extractable nuclear antigens (ENA)/dsDNA antibody testing was considered most informative.
Of the nuclear competent patterns, the centromere and homogeneous pattern obtained the highest scores for clinical relevance and the DFS pattern the lowest. Of the cytoplasmic patterns, the reticular/mitochondria-like pattern obtained the highest scores for clinical relevance and the polar/Golgi-like and rods and rings patterns the lowest.
Conclusion:
This survey confirms that the major nuclear and cytoplasmic ANA IIF patterns are considered clinically important. There is no unanimity on classifying DFS, rods and rings and polar/Golgi-like as a competent pattern and on reporting cytoplasmic patterns as ANA IIF positive
Current laboratory and clinical practices in reporting and interpreting anti-nuclear antibody indirect immunofluorescence (ANA IIF) patterns: results of an international survey
Background The International Consensus on Antinuclear Antibody (ANA) Patterns (ICAP) has recently proposed nomenclature in order to harmonize ANA indirect immunofluorescence (IIF) pattern reporting. ICAP distinguishes competent-level from expert-level patterns. A survey was organized to evaluate reporting, familiarity, and considered clinical value of ANA IIF patterns. Methods Two surveys were distributed by European Autoimmunity Standardization Initiative (EASI) working groups, the International Consensus on ANA Patterns (ICAP) and UK NEQAS to laboratory professionals and clinicians. Results 438 laboratory professionals and 248 clinicians from 67 countries responded. Except for dense fine speckled (DFS), the nuclear competent patterns were reported by > 85% of the laboratories. Except for rods and rings, the cytoplasmic competent patterns were reported by > 72% of laboratories. Cytoplasmic IIF staining was considered ANA positive by 55% of clinicians and 62% of laboratory professionals, with geographical and expertise-related differences. Quantification of fluorescence intensity was considered clinically relevant for nuclear patterns, but less so for cytoplasmic and mitotic patterns. Combining IIF with specific extractable nuclear antigens (ENA)/dsDNA antibody testing was considered most informative. Of the nuclear competent patterns, the centromere and homogeneous pattern obtained the highest scores for clinical relevance and the DFS pattern the lowest. Of the cytoplasmic patterns, the reticular/mitochondria-like pattern obtained the highest scores for clinical relevance and the polar/Golgi-like and rods and rings patterns the lowest. Conclusion This survey confirms that the major nuclear and cytoplasmic ANA IIF patterns are considered clinically important. There is no unanimity on classifying DFS, rods and rings and polar/Golgi-like as a competent pattern and on reporting cytoplasmic patterns as ANA IIF positive