90 research outputs found
Evidence of convective rolls in a sunspot penumbra
aims: We study the recently discovered twisting motion of bright penumbral
filaments with the aim of constraining their geometry and the associated
magnetic field. methods: A large sunspot located 40\degr from disk center was
observed at high resolution with the 1-m Swedish Solar Telescope. Inversions of
multi-wavelength polarimetric data and speckle reconstructed time series of
continuum images were used to determine proper motions, as well as the velocity
and magnetic structure in penumbral filaments. results: The continuum movie
reveals apparent lateral motions of bright and dark structures inside bright
filaments oriented parallel to the limb, confirming recent Hinode results. In
these filaments we measure upflows of on their
limbward side and weak downflows on their centerward side. The magnetic field
in them is significantly weaker and more horizontal than in the adjacent dark
filaments. conclusions: The data indicate the presence of vigorous convective
rolls in filaments with a nearly horizontal magnetic field. These are separated
by filaments harbouring stronger, more vertical fields. Because of reduced gas
pressure, we see deeper into the latter. When observed near the limb, the
disk-centerward side of the horizontal-field filaments appear bright due to the
\textit{hot wall} effect known from faculae. We estimate that the convective
rolls transport most of the energy needed to explain the penumbral radiative
flux.Comment: 4 pages, 4 figures, letter to Astronomy & Astrophysic
The low-mass Initial Mass Function in the 30 Doradus starburst cluster
We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations
of the central 56*57" (14pc*14.25pc) region around R136 in the starburst
cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to
derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test
whether the IMF in a massive metal-poor cluster is similar to that observed in
nearby young clusters and the field in our Galaxy. We estimate the mean age of
the cluster to be 3 Myr by combining our F160W photometry with previously
obtained HST WFPC2 optical F555W and F814W band photometry and comparing the
stellar locus in the color-magnitude diagram with main sequence and pre-main
sequence isochrones. The color-magnitude diagrams show the presence of
differential extinction and possibly an age spread of a few megayears. We
convert the magnitudes into masses adopting both a single mean age of 3 Myr
isochrone and a constant star formation history from 2 to 4 Myr. We derive the
IMF after correcting for incompleteness due to crowding. The faintest stars
detected have a mass of 0.5 Msun and the data are more than 50% complete
outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old
objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range
1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we
find no strong evidence for a flattening of the IMF down to 1.1 Msun at a
distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a
radius of 2 pc, reported in a previous optical HST study. We examine several
possible reasons for the different results. If the IMF determined here applies
to the whole cluster, the cluster would be massive enough to remain bound and
evolve into a relatively low-mass globular cluster.Comment: Accepted in ApJ. Abstract abridge
High-resolution spectro-polarimetry of a flaring sunspot penumbra
We present simultaneous photospheric and chromospheric observations of the
trailing sunspot in NOAA 10904 during a weak flare eruption (GOES magnitude
B7.8), obtained with the Swedish Solar Telescope (SST) in La Palma, Canary
Islands. High-resolution \ion{Ca}{ii} images show a typical two-ribbon
structure that has been hitherto only known for larger flares, and the flare
appears in a confined region that is discernible by a bright border. The
underlying photosphere shows a disturbed penumbral structure with intersecting
branches of penumbral filaments. High-resolution Doppler- and
vector-magnetograms exhibit oppositely directed Evershed flows and magnetic
field vectors in the individual penumbral branches, resulting in several
regions of magnetic azimuth discontinuity and several islands where the
vertical magnetic field is reversed. The discontinuity regions are co-spatial
with the locations of the onset of the flare ribbons. From the results, we
conclude that the confined flare region is detached from the global magnetic
field structure by a separatrix marked by the bright border visible in
\ion{Ca}{ii} . We further conclude that the islands of reversed vertical
field appear because of flux emergence and that the strong magnetic shear
appearing in the regions of magnetic azimuth discontinuity triggers the flare.Comment: 20 pages + 1 online Figure for A&
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
Modern optical astronomy: technology and impact of interferometry
The present `state of the art' and the path to future progress in high
spatial resolution imaging interferometry is reviewed. The review begins with a
treatment of the fundamentals of stellar optical interferometry, the origin,
properties, optical effects of turbulence in the Earth's atmosphere, the
passive methods that are applied on a single telescope to overcome atmospheric
image degradation such as speckle interferometry, and various other techniques.
These topics include differential speckle interferometry, speckle spectroscopy
and polarimetry, phase diversity, wavefront shearing interferometry,
phase-closure methods, dark speckle imaging, as well as the limitations imposed
by the detectors on the performance of speckle imaging. A brief account is
given of the technological innovation of adaptive-optics (AO) to compensate
such atmospheric effects on the image in real time. A major advancement
involves the transition from single-aperture to the dilute-aperture
interferometry using multiple telescopes. Therefore, the review deals with
recent developments involving ground-based, and space-based optical arrays.
Emphasis is placed on the problems specific to delay-lines, beam recombination,
polarization, dispersion, fringe-tracking, bootstrapping, coherencing and
cophasing, and recovery of the visibility functions. The role of AO in
enhancing visibilities is also discussed. The applications of interferometry,
such as imaging, astrometry, and nulling are described. The mathematical
intricacies of the various `post-detection' image-processing techniques are
examined critically. The review concludes with a discussion of the
astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics,
2002, to appear in April issu
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