543 research outputs found
The dusty Nebula surrounding HR Car: a Spitzer view
We present mid-IR observations of the Galactic Luminous Blue Variable (LBV)
HR Car and its associated nebula carried out with the Spitzer Space Telescope
using both IRAC and IRS, as part of a GTO program aimed to study stellar ejecta
from evolved stars. Our observations reveal a rich mid-IR spectrum of the inner
nebula showing both solid state and atomic gas signatures. Strong
low-excitation atomic fine structure lines such as m [\ion{Fe}{2}]
and m [\ion{Si}{2}], indicate, for the first time, the presence of a
PDR in this object class. While the physics and chemistry of the low-excitation
gas appears to be dominated by photodissociation, a possible contribution due
to shocks can be inferred from the evidence of gas phase Fe abundance
enhancement. The presence of amorphous silicates, inferred from the observed
characteristic broad feature at m located in the inner nebula, suggests
that dust has formed during the LBV outburst. This is in contrast with the
detection of crystalline dust in other probably more evolved Galactic LBVs,
which is similar to the crystalline dust observed in red supergiants. This has
been considered to be evidence of dust production during evolutionary phases
prior to the outburst.Comment: 27 pages, 6 figures. accepted by Ap
The Cuspy LINER Nucleus of the S0/a Galaxy NGC 2681
The nucleus of the bulge-dominated, multiply-barred S0/a galaxy NGC 2681 is
studied in detail, using high resolution Hubble Space Telescope FOC and NICMOS
imaging and FOS spectroscopy. The ionised gas central velocity dispersion is
found to increase by a factor ~2 when narrowing the aperture from R~1.5"
(ground) to R~0.1" (FOS). Dynamical modeling of these velocity dispersions
suggests that NGC 2681 does host a supermassive black hole (BH) for which one
can estimate a firm mass upper limit M_BH < 6*10^7 Solar Masses. This upper
limit is consistent with the relation between the central BH mass and velocity
dispersion M_BH - sigma known for other galaxies. The emission line ratios
place the nucleus of NGC 2681 among LINERs. It is likely that the emission line
region comes from a rather mild, but steady, feeding of gas to the central BH
in this galaxy. The inner stellar population lacks any measurable color
gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet,
consistently with FOC, FOS and IUE data, all indicating that this system
underwent a starburst ~1 Gyr ago that encompassed its whole interior, down to
its very center. The most likely source of such a widely-distributed starburst
is the dumping of tidally-extruded gas from a galaxy neighbor. If so, then NGC
2681 can be considered as the older brother of M82, seen face-on as opposed to
the edge-on view we have for M82.Comment: 25 pages, LaTeX, with 10 PostScript figures, to appear in The
Astrophysical Journa
Influence of activation protocol on perceived pain during rapid maxillary expansion
To investigate the influence of two different activation protocols on the timing and intensity of pain during rapid maxillary expansion (RME)
A Prediction of Brown Dwarfs in Ultracold Molecular Gas
A recent model for the stellar initial mass function (IMF), in which the
stellar masses are randomly sampled down to the thermal Jeans mass from
hierarchically structured pre-stellar clouds, predicts that regions of
ultra-cold CO gas, such as those recently found in nearby galaxies by Allen and
collaborators, should make an abundance of Brown Dwarfs with relatively few
normal stars. This result comes from the low value of the thermal Jeans mass,
considering that the hierarchical cloud model always gives the Salpeter IMF
slope above this lower mass limit. The ultracold CO clouds in the inner disk of
M31 have T~3K and pressures that are probably 10 times higher than in the solar
neighborhood. This gives a mass at the peak of the IMF equal to 0.01 Msun, well
below the Brown Dwarf limit of 0.08 Msun. Using a functional approximation to
the IMF, the ultracold clouds would have 50% of the star-like mass and 90% of
the objects below the Brown Dwarf limit. The brightest of the Brown Dwarfs in
M31 should have an apparent, extinction-corrected K-band magnitude of ~21 mag
in their pre-main sequence phase.Comment: 13 pages, 2 figures, to be published in Astrophysical Journal, Vol
522, September 10, 199
New Tests for Disruption Mechanisms of Star Clusters: The Large and Small Magellanic Clouds
We compare the observed bivariate distribution of masses(M) and ages(t) of
star clusters in the LMC with the predicted distributions g(M,t) from 3
idealized models for the disruption of star clusters: (1)sudden mass-dependent
disruption;(2)gradual mass-dependent disruption; and (3)gradual
mass-independent disruption. The model with mass-{\em in}dependent disruption
provides a good, first-order description of these cluster populations, with
g(M,t) propto M^{beta} t^{gamma}, beta=-1.8+/-0.2 and gamma=-0.8+/-0.2, at
least for clusters with ages t<10^9 yr and masses M<10^3 M_sol (more
specifically, t<10^7(M/10^2 M_sol)^{1.3} yr). This model predicts that the
clusters should have a power-law luminosity function, dN/dL propto L^-1.8, in
agreement with observations. The first two models, on the other hand, fare
poorly when describing the observations, refuting previous claims that
mass-dependent disruption of star clusters is observed in the LMC over the
studied M-t domain. Clusters in the SMC can be described by the same g(M,t)
distribution as for the LMC, but with smaller samples and hence larger
uncertainties. The successful g(M,t) model for clusters in the Magellanic
Clouds is virtually the same as the one for clusters in the merging Antennae
galaxies, but extends the domain of validity to lower masses and to older ages.
This indicates that the dominant disruption processes are similar in these very
different galaxies over at least t<10^8 yr and possibly t<10^9 yr. The mass
functions for young clusters in the LMC are power-laws, while that for ancient
globular clusters is peaked. We show that the observed shapes of these mass
functions are consistent with expectations from the simple evaporation model
presented by McLaughlin & Fall.Comment: 46 pages, 17 figures, published ApJ, vol 711, page 126
Diseño de viviendas de interés social : Su adecuación a las condiciones climáticas de Tucumán
El trabajo presenta la propuesta de diseño de viviendas de interés social desarrollada por el Instituto Provincial de la Vivienda y Desarrollo Urbano de Tucumán, en el marco del Proyecto GEF “Eficiencia energética y energías renovables en la vivienda social”. Se tuvo como premisa de diseño disminuir la demanda energética en las viviendas construidas por el Estado y se proyectaron respondiendo a pautas bioclimáticas adecuadas al sitio de emplazamiento. Se evaluó el comportamiento energético-ambiental de las propuestas comparativamente con las viviendas sociales convencionales. A partir de cálculos computacionales, ecuaciones simplificadas y métodos gráficos se analizó el comportamiento térmico, condiciones de asoleamiento y ventilación natural de las viviendas. Los resultados alcanzados demuestran que es posible mejorar sustancialmente el comportamiento energético de las viviendas a partir de un diseño eficiente, sin incrementar significativamente sus costos.The work presents the design proposal of social housing developed by the Provincial Institute of Housing and Urban Development (IPVyDU) of Tucuman, in the context of the GEF (Global Environment Facility) "Energy efficiency and renewable energies in social housing". The design is focused on reducing energy demand in homes built by the State and projected under patterns of adaptation in response to the climatic conditions of the proposed site. We evaluated the energyenvironmental behavior of the proposals in comparison with conventional proyects designed by IPVyDU. From computer calculations, simplified equations and graphical methods, we analyzed the thermal behavior, conditions of sunlight and natural ventilation of houses. The results obtained show that it is possible to substantially improve the energy performance of social housing from an efficient design without significantly increasing costs.Asociación Argentina de Energías Renovables y Medio Ambiente (ASADES
The red stellar population in NGC 1569
We present HST NICMOS photometry of the resolved stellar population in NGC
1569. The CMD in the F110W and F160W photometric bands contains ~2400 stars
with a formal photometric error < 0.1 mag down to J~23.5 and H~22.5. We
describe the data processing which is required to calibrate the instrumental
peculiarities of NICMOS. Two different packages for PSF-fitting photometry are
used to strengthen the photometric results in the crowded stellar field of NGC
1569. The resulting CMD is discussed in terms of the major evolutionary
properties of the resolved stellar populations. For a distance modulus of 26.71
and a reddening E(B-V)=0.56, our CMD samples stars down to ~0.8 Mo,
corresponding to look-back times > 15 Gyr. This is clear indication of SF
activity spanning an entire Hubble time. The metallicity of the reddest RGB
stars is in better agreement with Z=0.004 as measured in HII regions, than with
Z=0.0004 as expected from the stellar ages. The presence of - yet undetected -
very metal-poor stars embedded in the stellar distribution around J=22.75 and
J-H=1.15 is, however, not ruled out. The youngest stars (< 50 Myr) are
preferentially found around the two central super star clusters, whereas the
oldest population has a more uniform spatial distribution. A SFR per unit area
of 1 Mo yr*(-1) kpc*(-2) and a mass formed in stars of ~ 1.4x10*6 Mo in the
last 50 Myr are derived from the CMD. The NIR CMD places strong constraints on
the lower limit of the onset of SF in NGC 1569. The exceptionally high crowding
in the NICMOS images of NGC 1569 is a challenge for the photometric analysis.
As a result, optical and NIR images of NGC 1569 sample different populations
and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of
the SF histories derived from the optical and NIR CMDs.Comment: 41 pages including 1 table (Latex) and 14 figures (postscript).
Accepted for publication in the Astronomical Journal, March 2001 issu
Polarimetric Evidence of Non-Spherical Winds
Polarization observations yield otherwise unobtainable information about the
geometrical structure of unresolved objects. In this talk we review the
evidences for non-spherically symmetric structures around Luminous Hot Stars
from polarimetry and what we can learn with this technique. Polarimetry has
added a new dimension to the study of the envelopes of Luminous Blue Variables,
Wolf-Rayet stars and B[e] stars, all of which are discussed in some detail.Comment: 8 pages, 2 encapsulated Postscript figures, uses lamuphys.sty.
Invited review to appear in IAU Coll. 169, Variable and Non-Spherical Stellar
Winds in Luminous Hot Stars, eds. B. Wolf, A.Fullerton and O. Stahl
(Springer
The NICMOS Snapshot Survey of nearby Galaxies
We present ``snapshot'' observations with the NearInfrared Camera and
MultiObject Spectrometer (NICMOS) on board the Hubble Space Telescope (HST) of
94 nearby galaxies from the Revised Shapley Ames Catalog. Images with 0.2 as
resolution were obtained in two filters, a broad-band continuum filter (F160W,
roughly equivalent to the H-band) and a narrow band filter centered on the
Paschen alpha line (F187N or F190N, depending on the galaxy redshift) with the
51x51 as field of view of the NICMOS camera 3. A first-order continuum
subtraction is performed, and the resulting line maps and integrated Paschen
alpha line fluxes are presented. A statistical analysis indicates that the
average Paschen alpha surface brightness {\bf in the central regions} is
highest in early-type (Sa-Sb) spirals.Comment: Original contained error in flux calibration. Table 1 now has correct
Paschen Alpha fluxes. 14 pages LaTeX with JPEG and PS figures. Also available
at http://icarus.stsci.edu/~boeker/publications.htm
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