1,768 research outputs found
NIR spectroscopy of the most massive open cluster in the Galaxy: Westerlund 1
Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars
in the central 2'x2' of the cluster Westerlund 1 (Wd 1) with a resolution of
R~9000 at a central wavelength of 2.30 micron. This allowed us to determine
radial velocities of ten post-main-sequence stars, and from these values a
velocity dispersion. Assuming virial equilibrium, the dispersion of sigma=8.4
km/s leads to a total dynamical cluster mass of 1.25x10^5 solar masses,
comparable to the photometric mass of the cluster. There is no extra-virial
motion which would have to be interpreted as a signature of cluster expansion
or dissolution.Comment: To appear in the proceedings of IAU 246: "Dynamical Evolution of
Dense Stellar Systems" (E. Vesperini, M. Giersz, A. Sills, eds.
On the Relative Power of Reduction Notions in Arithmetic Circuit Complexity
We show that the two main reduction notions in arithmetic circuit complexity, p-projections and c-reductions, differ in power. We do so by showing unconditionally that there are polynomials that are VNP-complete under c-reductions but not under p-projections. We also show that the question of which polynomials are VNP-complete under which type of reductions depends on the underlying field
On the Relative Power of Reduction Notions in Arithmetic Circuit Complexity
We show that the two main reduction notions in arithmetic circuit complexity, p-projections and c-reductions, differ in power. We do so by showing unconditionally that there are polynomials that are VNP-complete under c-reductions but not under p-projections. We also show that the question of which polynomials are VNP-complete under which type of reductions depends on the underlying field
Near-Infrared-Spectroscopy with Extremely Large Telescopes: Integral-Field- versus Multi-Object-Instruments
Integral-field-spectroscopy and multi-object-spectroscopy provide the high
multiplex gain required for efficient use of the upcoming generation of
extremely large telescopes. We present instrument developments and designs for
both concepts, and how these designs can be applied to cryogenic near-infrared
instrumentation. Specifically, the fiber-based concept stands out the
possibility to expand it to any number of image points, and its modularity
predestines it to become the new concept for multi-field-spectroscopy. Which of
the three concepts --- integral-field-, multi-object-, or
multi-field-spectroscopy --- is best suited for the largest telescopes is
discussed considering the size of the objects and their density on the sky.Comment: 8 pages, 4 figures (converted to bitmap), to appear in the
proceedings of the Workshop on Extremely Large Telescopes, Sweden, June 1-2,
1999, uses spie.sty (V0.91) and spiebib.bst (V0.91
How Universal are the Young Cluster Sequences? - the Cases of LMC, SMC, M83 and the Antennae
Aims.Recently a new analysis of cluster observations in the Milky Way found
evidence that clustered star formation may work under tight constraints with
respect to cluster size and density, implying the presence of just two
sequences of young massive cluster. These two types of clusters each expand at
different rates with cluster age. Methods. Here we investigate whether similar
sequences exist in other nearby galaxies. Results:We find that while for the
extragalactic young stellar clusters the overall trend in the cluster-density
scaling is quite comparable to the relation obtained for Galactic clusters,
there are also possible difference. For the LMC and SMC clusters the densities
are below the Galactic data points and/or the core radii are smaller than those
of data points with comparable density. For M83 and the Antenna clusters the
core radii are possibly comparable to the Galactic clusters but it is not clear
whether they exhibit similar expansion speeds. These findings should serve as
an incentive to perform more systematic observations and analysis to answer the
question of a possible similarity between young galactic and extragalactic star
clusters sequences.Comment: 6 pages, 4 figures, A&A in pres
Cluster and nebular properties of the central star-forming region of NGC 1140
We present new high spatial resolution HST/ACS imaging of NGC 1140 and high
spectral resolution VLT/UVES spectroscopy of its central star-forming region.
The central region contains several clusters, the two brightest of which are
clusters 1 and 6 from Hunter, O'Connell & Gallagher, located within
star-forming knots A and B, respectively. Nebular analysis indicates that the
knots have an LMC-like metallicity of 12 + log(O/H) = 8.29 +/- 0.09. According
to continuum subtracted H alpha ACS imaging, cluster 1 dominates the nebular
emission of the brighter knot A. Conversely, negligible nebular emission in
knot B originates from cluster 6. Evolutionary synthesis modelling implies an
age of 5 +/- 1 Myr for cluster 1, from which a photometric mass of (1.1 +/-
0.3) x 10^6 Msun is obtained. For this age and photometric mass, the modelling
predicts the presence of ~5900 late O stars within cluster 1. Wolf-Rayet
features are observed in knot A, suggesting 550 late-type WN and 200 early-type
WC stars. Therefore, N(WR)/N(O) ~ 0.1, assuming that all the WR stars are
located within cluster 1. The velocity dispersions of the clusters were
measured from constituent red supergiants as sigma ~ 23 +/- 1 km/s for cluster
1 and sigma ~ 26 +/- 1 km/s for cluster 6. Combining sigma with half-light
radii of 8 +/- 2 pc and 6.0 +/- 0.2 pc measured from the F625W ACS image
implies virial masses of (10 +/- 3) x 10^6 Msun and (9.1 +/- 0.8) x 10^6 Msun
for clusters 1 and 6, respectively. The most likely reason for the difference
between the dynamical and photometric masses of cluster 1 is that the velocity
dispersion of knot A is not due solely to cluster 1, as assumed, but has an
additional component associated with cluster 2.Comment: 13 pages, 7 figure
A new era of spectroscopy: SINFONI, NIR integral field spectroscopy at the diffraction limit of an 8m telescope
SINFONI, the SINgle Faint Object Near-infrared Investigation, is an
instrument for the Very Large Telescope (VLT), which will start its operation
mid 2002 and allow for the first time near infrared (NIR) integral field
spectroscopy at the diffraction limit of an 8-m telescope. SINFONI is the
combination of two state-of-the art instruments, the integral field
spectrometer SPIFFI, built by the Max-Planck-Institut fuer extraterrestrische
Physik (MPE), and the adaptive optics (AO) system MACAO, built by the European
Southern Observatory (ESO). It will allow a unique type of observations by
delivering simultaneously high spatial resolution (pixel sizes 0.025arcsec to
0.25arcsec) and a moderate spectral resolution (R~2000 to R~4500), where the
higher spectral resolution mode will allow for software OH suppression. This
opens new prospects for astronomy.Comment: 9 pages, 4 figures, to appear in SPIE proceedings "Astronomical
Telescopes and Instrumentation 2000". More recent sensitivity estimates are
available at http://www.mpe.mpg.de/www_ir/ir_instruments/sinfoni/spiffi.ht
Surface differential rotation and prominences of the Lupus post T Tauri star RX J1508.6-4423
We present in this paper a spectroscopic monitoring of the Lupus post T Tauri star RX J1508.6-4423 carried out at two closely separated epochs (1998 May 06 and 10) with the UCL Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Applying least-squares convolution and maximum entropy image reconstruction techniques to our sets of spectra, we demonstrate that this star features on its surface a large cool polar cap with several appendages extending to lower latitudes, as well as one spot close to the equator. The images reconstructed at both epochs are in good overall agreement, except for a photospheric shear that we interpret in terms of latitudinal differential rotation. Given the spot distribution at the epoch of our observations, differential rotation could only be investigated between latitudes 15° and 60°. We find in particular that the observed differential rotation is compatible with a solar-like law (i.e., with rotation rate decreasing towards high latitudes proportionally to sin 2l, where l denotes the latitude) in this particular latitude range. Assuming that such a law can be extrapolated to all latitudes, we find that the equator of RX J1508.6-4423 does one more rotational cycle than the pole every 50 ±10 d, implying a photospheric shear 2 to 3 times stronger than that of the Sun. We also discover that the Hα emission profile of RX J1508.6-4423 is most of the time double-peaked and strongly modulated with the rotation period of the star. We interpret this rotationally modulated emission as being caused by a dense and complex prominence system, the circumstellar distribution of which is obtained through maximum entropy Doppler tomography. These maps show in particular that prominences form a complete and inhomogeneous ring around the star, precisely at the corotation radius. We use the total Hα and HÎČ emission flux to estimate that the mass of the whole prominence system is about 10 20g. From our observation that the whole cloud system surrounding the star is regenerated in less than 4 d, we conclude that the braking time-scale of RX J1508.6-4423 is shorter than 1 Gyr, and that prominence expulsion is thus likely to contribute significantly to the rotational spindown of young low-mass stars
Medium resolution 2.3 micron spectroscopy of the massive Galactic open cluster Westerlund 1
The Galactic open cluster Westerlund 1 was found only a few years ago to be
much more massive than previously thought, with evidence suggesting its mass to
be in excess of ~10^5 Msun, in the range spanned by young extragalactic star
clusters. Unlike those clusters its proximity makes spatially resolved studies
of its stellar population feasible. It is therefore the ultimate template for a
young, massive star cluster, permitting direct comparison of its properties
with measurements of velocity dispersion and dynamical mass for spatially
unresolved extragalactic clusters.
To this end, we used the long slit near-infrared spectrograph VLT/ISAAC to
observe the CO bandhead region near 2.29 micron scanning the slit across the
cluster centre during the integration. Spatially collapsing the spectra along
the slit results in a single co-added spectrum of the cluster, comparable to
what one would obtain in the extragalactic cluster context.
This spectrum was analysed the same way as the spectra of almost point-like
extragalactic clusters, using red superiant cluster members as velocity
templates.
We detected four red supergiants which are included in the integrated
spectrum, and our measured velocity dispersion is 5.8 km/s. Together with the
cluster size of 0.86 pc, derived from archival near-infrared SOFI-NTT images,
this yields a dynamical mass of 6.3x10^4 Msun. While this value is not to be
considered the final word, there is at least so far no sign for rapid expansion
or collapse.Comment: 6 pages, 5 figures, A&A accepte
A Triarylamine-Triarylborane Dyad with a Photochromic Dithienylethene Bridge
A molecular triad composed of a triarylamine donor, a triarylborane acceptor, and a photoisomerizable dithienylethene bridge has been synthesized and explored by cyclic voltammetry, UVâvis, and luminescence spectroscopy. The effects of irradiation with UV light and fluoride addition on the electrochemical and optical spectroscopic properties of the donorâbridgeâacceptor molecule were investigated. Photoisomerization of the dithienylethene bridge affects the triarylboron reduction potential, but not the triarylamine oxidation potential. UVâvis experiments reveal that the association constant for fluoride binding at the triarylborane site is independent of the isomerization state of the bridge. Irradiation of a THF solution of our donorâbridgeâacceptor molecule with UV light, followed by Fâ addition, leads to a different color of the sample than UV irradiation alone or Fâ addition alone
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