426 research outputs found
Reduction and analysis techniques for infrared imaging data
Infrared detector arrays are becoming increasingly available to the astronomy community, with a number of array cameras already in use at national observatories, and others under development at many institutions. As the detector technology and imaging instruments grow more sophisticated, more attention is focussed on the business of turning raw data into scientifically significant information. Turning pictures into papers, or equivalently, astronomy into astrophysics, both accurately and efficiently, is discussed. Also discussed are some of the factors that can be considered at each of three major stages; acquisition, reduction, and analysis, concentrating in particular on several of the questions most relevant to the techniques currently applied to near infrared imaging
Binary stars in young clusters: models versus observations of the Trapezium Cluster
The frequency of low-mass pre-main sequence binary systems is significantly
lower in the Trapezium Cluster than in Taurus-Auriga. We investigate if this
difference can be explained through stellar encounters in dense clusters. To
this effect, a range of possible models of the well observed Trapezium Cluster
are calculated using Aarseth's direct N-body code, which treats binaries
accurately. The results are confronted with observational constraints. The
range of models include clusters in virial equilibrium, expanding clusters as a
result of instantaneous mass loss, as well as collapsing clusters. In all cases
the primordial binary proportion is larger than 50 per cent, with initial
period distributions as observed in Taurus-Auriga and the Galactic field.
It is found that the expanding model, with an initial binary population as in
the Galactic field, is most consistent with the observational constraints. This
raises the possibility that the primordial group of OB stars may have expelled
the cluster gas roughly 50 000 yr ago. The cluster's bulk expansion rate is
thus a key observable that needs to be determined. The other models demonstrate
that the rapidly decreasing binary proportion, its radial dependence and the
form of the period distribution, together with structural and kinematical data,
are very useful diagnostics on the present and past dynamical state of a young
cluster. In particular, kinematical cooling from the disruption of wide
binaries is seen for the first time.Comment: 34 pages, 13 figures, LaTeX, uses own_sngl.sty, accepted by New
A NICMOS Direct Imaging Search for Giant Planets around the Single White Dwarfs in the Hyades
We report preliminary results from our search for massive giant planets (6-12
Jupiter masses) around the known seven single white dwarfs in the Hyades
cluster at sub-arcsec separations. At an age of 625 Myr, the white dwarfs had
progenitor masses of about 3 solar masses, and massive gaseous giant planets
should have formed in the massive circumstellar disks around these ex-Herbig A0
stars, probably at orbital separations similar or slightly larger than that of
Jupiter. Such planets would have survived the post-Main-Sequence mass loss of
the parent star and would have migrated outward adiabatically to a distance of
about 25 AU. At the distance of the Hyades (45 pc) this corresponds to an
angular separation of 0.5 arcsec. J and H magnitudes of these giants are in the
range of 20.5-23.3 mag, which can be resolved with NICMOS. The achieved
sensitivities and contrast ratios agree well with simulations. Preliminary
evaluation of the NICMOS data set did not reveal any evidence for neither
planetary mass companions with masses down to about 10 Jupiter masses nor brown
dwarfs around any of the seven white dwarfs for separations larger than 0.5
arcsec.Comment: 14th European Workshop on White Dwarf
SSSPM J1444-2019: an extremely high proper motion, ultracool subdwarf
We present the discovery of a new extreme high proper motion object (3.5
arcsec/year) which we classify as an ultracool subdwarf with [M/H] = -0.5. It
has a formal spectral type of sdM9 but also shows L-type features: while the VO
bands are completely absent, it exhibits extremely strong TiO absorption in its
optical spectrum. With a radial velocity of about -160 km/s and a rough
distance estimate of 16--24 pc, it is likely one of the nearest halo members
crossing the Solar neighbourhood with a heliocentric space velocity of
(U,V,W)=(-244,-256,-100)+/-(32,77,6) km/s.Comment: 4 pages, 4 figures (Fig.1a-d available as jpg files), accepted for
publication in Astronomy & Astrophysics Letter
Cluster-assisted accretion for massive stars
Gravitational interactions in very young high-density stellar clusters can to
some degree change the angular momentum in the circumstellar discs surrounding
initially the majority of stars. However, for most stars the cluster
environment alters the angular momentum only slightly. For example, in
simulations of the Orion Nebula cluster (ONC) encounters reduce the angular
momentum of the discs on average at most by 3-5% and in the higher density
region of the Trapezium %where encounters are more likely, the disc angular
momentum is on average lowered by 15-20% - still a minor loss process. However,
in this paper it is demonstrated that the situation is very different if one
considers high-mass stars (M* > 10 M(solar) only. Assuming an age of 2 Myr for
the ONC, their discs have on average a 50-90% lower angular momentum than
primordially. This enormous loss in angular momentum in the disc should result
in an equivalent increase in accretion, implying that the cluster environment
boosts accretion for high-mass stars, thus %in the cluster center, making them
even more massive.Comment: 10 pages including 2 figures, accepted for publication in ApJ
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