82 research outputs found
The Link Between UV Extinction and Infrared Cirrus
Low resolution spectra from the International Ultraviolet Explorer satellite were used to derive ultraviolet extinction curves for stars in four clusters away from the galactic plane. The extinction in three of the clusters is very similar to the general interstellar curve defined by Seaton. Stars in the fourth region, near the Rho Ophiuci dark cloud, have extinction curves that are characterized by a small "linear" term component. The star BD +36 deg 781 is unique amongst the 20 stars observed in that it shows evidence for extinction by diamond grains near 1700 angstroms. We used data from the final release of the IRAS Sky Survey Atlas (ISSA) to determine the 60 micron to 100 micron intensity ratio for the infrared cirrus. The ISSA data, which have been corrected for zodiacal light, gave intensity ratios that are more robust and self-consistent than for other data sets that we used. When the infrared and ultraviolet data are combined, we see a general trend for low values of the ultraviolet "linear term" (al) to correlate with high values of 60 micron/100 micron ratio. This implies that, in regions where the average dust temperature is hotter (high 60 micron/100 micron ratio), there is a relative absence of the small silicate grains that are responsible for the ultraviolet linear term. However, the new data do not bear out our earlier contention that the 60 micron and 100 micron emissions are poorly correlated spatially in regions where the 60 micron/100 micron ratio is low. Only NGC 1647 shows this result. It may be that the different dust types are particularly poorly mixed in this area
An Infrared Color-Magnitude Relationship
We have investigated a sample of dusty supergiants and Mira variables and have found a roughly linear relationship between the absolute magnitude at 12 mum and the [l2]-[25] color. Both samples follow the same infrared color-magnitude relationship, whic
High spatial-resolution IRAS images of M51
High spatial-resolution (approx. equal to 30 seconds) images of M51 in the four Infrared Astronomy Satellite (IRAS) bands (12, 25, 60, and 100 microns) have been obtained. The spatial variation in flux in all four bands is coincident with the spiral features seen in H alpha and 6 cm with a few exceptions. In the nuclear region (4 minutes) the position of the peak of maximum intensity shifts in relation to the visual nucleus: it is coincident with the nucleus at 12 microns, shifts approximately 45 seconds to south-southwest, and is 45 seconds northwest of the nucleus at 60 and 100 microns
Improved to Transformation Equations for Main Sequence Stars
We report improved transformation equations between the and
photometric systems. Although the details of the transformations
depend on luminosity class, we find a typical rms scatter on the order of 0.001
magnitude if the sample is limited to main sequence stars. Furthermore, we find
an accurate transformation requires complex, multi-color dependencies for the
bluer bandpasses. Results for giant stars will be reported in a subsequent
paper.Comment: 7 pages, 8 figure
Theoretical stellar models for old galactic clusters
We present new evolutionary stellar models suitable for old Population I
clusters, discussing both the consequences of the most recent improvements in
the input physics and the effect of element diffusion within the stellar
structures. Theoretical cluster isochrones are presented, covering the range of
ages from 1 to 9 Gyr for the four selected choices on the metallicity Z= 0.007,
0.010, 0.015 and 0.020. Theoretical uncertainties on the efficiency of
superadiabatic convection are discussed in some details. Isochrone fitting to
the CM diagrams of the two well observed galactic clusters NGC2420 and M67
indicates that a mixing length parameter alpha = 1.9 appears adequate for
reproducing the observed color of cool giant stars. The problems in matching
theoretical preditions to the observed slope of MS stars are discussed.Comment: 7 pages, 14 postscript figures, accepted for publication on MNRA
High spatial resolution restoration of IRAS images
A general technique to improve the spatial resolution of the IRAS AO data was developed at The Aerospace Corporation using the Maximum Entropy algorithm of Skilling and Gull. The technique has been applied to a variety of fields and several individual AO MACROS. With this general technique, resolutions of 15 arcsec were achieved in 12 and 25 micron images and 30 arcsec in 60 and 100 micron images. Results on galactic plane fields show that both photometric and positional accuracy achieved in the general IRAS survey are also achieved in the reconstructed images
Statistics of Stellar Populations of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud
A comparative analysis of Washington color-magnitude diagrams (CMDs) for 14
star clusters and respective surrounding fields in the Large Magellanic Cloud
(LMC) outer disk is presented. Each CCD frame including field and respective
cluster covers an area of 185 arcmin^2. The stellar population sampled is of
intermediate age and metallicity. CMD radial analysis involving star count
ratios, morphology and integrated light properties are carried out. Luminosity
functions (LFs) are also presented. Two main results are: (i) Within the range
4<R(kpc)<8, the distance from the LMC center is well correlated with the
average age in the sense that inner fields are younger and; (ii) Beyond
approximately 8kpc the outer fields do not show evidence of a significant
intermediate-age component in their stellar populations, as inferred from red
giant clump star counts.Comment: 27 pages, 4 tables, 11 figures; accepted by the A
The local stellar population of nova regions in the Large Magellanic Cloud
This study aims at identifying and understanding the parent population of
novae in the Large Magellanic Cloud (LMC) by studying the local, projected,
stellar population. The star formation history of the local environment around
novae is studied based on photometric data of stars and star clusters in the
nova neighbourhood, available in the OGLE II survey and star cluster
catalogues. The ages of stellar population within a few arcmin around novae
regions are estimated using isochrone fits to the V vs (V-I) colour-magnitude
diagrams. The fraction of stars in various evolutionary states are compared
using luminosity functions of the main-sequence stars and the red giant stars.
The age, density and luminosity function of the stellar population are
estimated around 15 novae. The upper limit of the age of the intermediate
stellar population is found to be 4 Gyr in all the regions, excepting the
region around the slow nova LMC 1948. Star formation in these regions is found
to have started between 4 - 2.0 Gyr, with a majority of the regions starting
the star formation at 3.2 Gyr. This star formation event lasted until 1.6 - 0.8
Gyr The star formation history of the underlying population of both the fast
and moderately fast novae indicate their parent population to be similar and
likely to be in the age range 3.2 - 1.0 Gyr. This is in good agreement with the
theoretical age estimates for Galactic cataclysmic variables. The region around
the slow nova shows a stellar population in the age range 1 - 10 Gyr, with a
good fraction belonging to an older population, consistent with the idea that
the progenitors of slow novae belong to older population.Comment: to appear in A&A (final version - error analysis included, typos
corrected, figures 17 and 18 changed
Integrated Colours of Milky Way Globular Clusters and Horizontal Branch Morphology
Broadband colours are often used as metallicity proxies in the study of
extragalactic globular clusters. A common concern is the effect of variations
in horizontal branch (HB) morphology--the second-parameter effect--on such
colours. We have used UBVI, Washington, and DDO photometry for a compilation of
over 80 Milky Way globular clusters to address this question. Our method is to
fit linear relations between colour and [Fe/H], and study the correlations
between the residuals about these fits and two quantitative measures of HB
morphology. While there is a significant HB effect seen in U-B, for the
commonly used colours B-V, V-I, and C-T_1, the deviations from the baseline
colour-[Fe/H] relations are less strongly related to HB morphology. There may
be weak signatures in B-V and C-T_1, but these are at the limit of
observational uncertainties. The results may favour the use of B-I in studies
of extragalactic globular clusters, especially when its high [Fe/H]-sensitivity
is considered.Comment: 19 pages, including 26 figures. AN in press. Figure 9 stubbornly
resists attempts to correct i
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