13 research outputs found
The uBVI Photometric System. I. Motivation, Implementation, and Calibration
This paper describes the design principles for a CCD-based photometric system
that is highly optimized for ground-based measurement of the size of the Balmer
jump in stellar energy distributions. It is shown that, among ultraviolet
filters in common use, the Thuan-Gunn u filter is the most efficient for this
purpose. This filter is combined with the standard Johnson-Kron-Cousins B, V,
and I bandpasses to constitute the uBVI photometric system.
Model stellar atmospheres are used to calibrate color-color diagrams for the
uBVI system in terms of the fundamental stellar parameters of effective
temperature, surface gravity, and metallicity. The u-B index is very sensitive
to log g, but also to [Fe/H]. It is shown that an analog of the Str\"omgren c_1
index, defined as (u-B)-(B-V), is much less metallicity dependent, but still
sensitive to log g. The effect of interstellar reddening on u-B is determined
through synthetic photometric calculations, and practical advice is given on
dealing with flat fields, atmospheric extinction, the red leak in the u filter,
and photometric reductions.
The uBVI system offers a wide range of applicability in detecting stars of
high luminosity in both young (yellow supergiants) and old (post-AGB stars)
populations, using stars of both types as standard candles to measure
extragalactic distances with high efficiency, and in exploring the horizontal
branch in globular clusters. In many stellar applications, it can profitably
replace the classical uBVI system.
Paper II in this series will present a network of well-calibrated standard
stars for the uBVI system.Comment: 24 pages, 5 figures, 4 tables; accepted by A
All-Sky spectrally matched UBVRI-ZY and u'g'r'i'z' magnitudes for stars in the Tycho2 catalog
We present fitted UBVRI-ZY and u'g'r'i'z' magnitudes, spectral types and
distances for 2.4M stars, derived from synthetic photometry of a library
spectrum that best matches the Tycho2 BtVt, NOMAD Rn and 2MASS JHK_{2/S}
catalog magnitudes. We present similarly synthesized multi-filter magnitudes,
types and distances for 4.8M stars with 2MASS and SDSS photometry to g<16
within the Sloan survey region, for Landolt and Sloan primary standards, and
for Sloan Northern (PT) and Southern secondary standards.
The synthetic magnitude zeropoints for BtVt, UBVRI, ZvYv, JHK_{2/S},
JHK_{MKO}, Stromgren uvby, Sloan u'g'r'i'z' and ugriz are calibrated on 20
calspec spectrophotometric standards. The UBVRI and ugriz zeropoints have
dispersions of 1--3%, for standards covering a range of color from -0.3 < V-I <
4.6; those for other filters are in the range 2--5%.
The spectrally matched fits to Tycho2 stars provide estimated 1-sigma errors
per star of ~0.2, 0.15, 0.12, 0.10 and 0.08 mags respectively in either UBVRI
or u'g'r'i'z'; those for at least 70% of the SDSS survey region to g<16 have
estimated 1-sigma errors per star of ~0.2, 0.06, 0.04, 0.04, 0.05 in u'g'r'i'z'
or UBVRI.
The density of Tycho2 stars, averaging about 60 stars per square degree,
provides sufficient stars to enable automatic flux calibrations for most
digital images with fields of view of 0.5 degree or more. Using several such
standards per field, automatic flux calibration can be achieved to a few
percent in any filter, at any airmass, in most workable observing conditions,
to facilitate inter-comparison of data from different sites, telescopes and
instruments.Comment: 36 pages, 30 figures, 3 printed tables, several electronic tables,
accepted PASP Dec 201
Determining the Physical Properties of the B Stars I. Methodology and First Results
We describe a new approach to fitting the UV-to-optical spectra of B stars to
model atmospheres and present initial results. Using a sample of lightly
reddened stars, we demonstrate that the Kurucz model atmospheres can produce
excellent fits to either combined low dispersion IUE and optical photometry or
HST FOS spectrophotometry, as long as the following conditions are fulfilled:
1) an extended grid of Kurucz models is employed,
2) the IUE NEWSIPS data are placed on the FOS absolute flux system using the
Massa & Fitzpatrick (1999) transformation, and
3) all of the model parameters and the effects of interstellar extinction are
solved for simultaneously.
When these steps are taken, the temperatures, gravities, abundances and
microturbulence velocities of lightly reddened B0-A0 V stars are determined to
high precision. We also demonstrate that the same procedure can be used to fit
the energy distributions of stars which are reddened by any UV extinction curve
which can be expressed by the Fitzpatrick & Massa (1990) parameterization
scheme.
We present an initial set of results and verify our approach through
comparisons with angular diameter measurements and the parameters derived for
an eclipsing B star binary. We demonstrate that the metallicity derived from
the ATLAS 9 fits to main sequence B stars is essentially the Fe abundance. We
find that a near zero microturbulence velocity provides the best-fit to all but
the hottest or most luminous stars (where it may become a surrogate for
atmospheric expansion), and that the use of white dwarfs to calibrate UV
spectrophotometry is valid.Comment: 17 pages, including 2 pages of Tables and 6 pages of Figures.
Astrophysical Jounral, in pres
A Study of the B-V Colour Temperature Relation
We attempt to construct a B-V colour temperature relation for stars in the
least model dependent way employing the best modern data. The fit we obtained
with the form Teff = Teff((B-V)0,[Fe/H],log g) is well constrained and a number
of tests show the consistency of the procedures for the fit. Our relation
covers from F0 to K5 stars with metallicity [Fe/H] = -1.5 to +0.3 for both
dwarfs and giants. The residual of the fit is 66 K, which is consistent with
what are expected from the quality of the present data. Metallicity and surface
gravity effects are well separated from the colour dependence. Dwarfs and
giants match well in a single family of fit, differing only in log g. The fit
also detects the Galactic extinction correction for nearby stars with the
amount E(B-V) = 0.26 +/-0.03 mag/kpc. Taking the newly obtained relation as a
reference we examine a number of B-V colour temperature relations and
atmosphere models available in the literature. We show the presence of a
systematic error in the colour temperature relation from synthetic calculations
of model atmospheres; the systematic error across K0 to K5 dwarfs is 0.04-0.05
mag in B-V, which means 0.25-0.3 mag in Mv for the K star range. We also argue
for the error in the temperature scale used in currently popular stellar
population synthesis models; synthetic colours from these models are somewhat
too blue for aged elliptical galaxies. We derive the colour index of the sun
(B-V)sun = 0.627 +/-0.018, and discuss that redder colours (e.g., 0.66-0.67)
often quoted in the literature are incompatible with the colour-temperature
relation.Comment: AASLaTeX (aaspp4.sty),36 pages (13 figures included), submitted to
Astronomical Journal, replaced (typo in author name
The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance
We present a map of the spiral structure of the Galaxy, as traced by
molecular CS emission associated with IRAS sources which are believed to be
compact HII regions. The CS line velocities are used to determine the kinematic
distances of the sources, in order to investigate their distribution in the
galactic plane. This allows us to use 870 objects to trace the arms, a number
larger than that of previous studies based on classical HII regions. The
distance ambiguity of the kinematic distances, when it exists, is solved by
different procedures, including the latitude distribution and an analysis of
the longitude-velocity diagram. The well defined spiral arms are seen to be
confined inside the co-rotation radius, as is often the case in spiral
galaxies. We identify a square-shaped sub-structure in the CS map with that
predicted by stellar orbits at the 4:1 resonance (4 epicycle oscillations in
one turn around the galactic center). The sub-structure is found at the
expected radius, based on the known pattern rotation speed and epicycle
frequency curve. An inner arm presents an end with strong inward curvature and
intense star formation that we tentatively associate with the region where this
arm surrounds the extremity of the bar, as seen in many barred galaxies.
Finally, a new arm with concave curvature is found in the Sagitta to Cepheus
region of the sky
Why Hot Horizontal Branch Stars Can Appear Redder Than Red Giants
In this paper we report on a curious feature in the V, (U-B) color-magnitude
diagrams of globular clusters. In our database, we find that a considerable
fraction of blue horizontal branch stars, hotter than the instability strip and
cooler than the Grundahl et al. (1999) jump (i.e., 6000 <~ T_eff K <~ 10000),
have (U-B) colors redder than their red giant progenitors. This red incursion
is not expected on theoretical grounds, as horizontal branch stars (whose
convective regions are less extended than in red giant structures) should not
``appear'' cooler than a red giant. Analyzing data from different telescopes we
show that: 1) the horizontal branch red incursion is strongly dependent on the
shape of the adopted U filter and to a lesser extent, on the B filter; 2) the
photometry done with U filters which do not encompass the Balmer jump, do not
show the blue horizontal branch red incursion; 3) the occurrence of this
feature is also due to the peculiar dependence of the U and B magnitudes on
stars effective temperature, gravity, and metallicity; 4) theoretical tracks
can reproduce the observed horizontal branch morphology, provided that the
appropriate (i.e. exactly responding to the filters effectively used in the
observations) transmission curve efficiencies are used for deriving color-T_eff
transformations; 5) the red incursion extent depends on metallicity.Comment: 7 pages, 6 figures, Astronomy and Astrophysic, accepted May 15 200
A Parameter Study of Type II Supernova Light Curves Using 6 M_odot He Cores
Results of numerical calculations of Type II supernova light curves are
presented. The model progenitor stars have 6 cores and various
envelopes, originating from a numerically evolved 20 star. Five
parameters that affect the light curves are examined: the ejected mass, the
progenitor radius, the explosion energy, the Ni mass, and the extent of
Ni mixing. The following affects have been found: 1) the larger the
progenitor radius the brighter the early--time light curve, with little affect
on the late--time light curve, 2) the larger the envelope mass the fainter the
early light curve and the flatter the slope of the late light curve, 3) the
larger the explosion energy the brighter the early light curve and the steeper
the slope of the late light curve, 4) the larger the Ni mass the
brighter the overall light curve after 20 to 50 days, with no affect on the
early light curve, 5) the more extensive the Ni mixing the brighter the
early light curve and the steeper the late light curve. The primary parameters
affecting the light curve shape are the progenitor radius and the ejected mass.
The secondary parameters are the explosion energy, Ni mass and Ni
mixing. I find that while in principle the general shape and absolute magnitude
of a light curve indicate a unique set of parameters, in practice it is
difficult to avoid some ambiguity in the parameters. I find that the
nickel--powered diffusion wave and the recombination of helium produce a
prominent secondary peak in all our calculations. The feature is less prominent
when compositional mixing, both Ni mixing and mixing between the
hydrogen and helium layers, occurs. The model photospheric temperatures and
velocities are presented, for comparison to observation.Comment: 39 pages, 15 figures. Astrophysical Journal (Accepted, Dec. 20, 2004
A large stellar evolution database for population synthesis studies. I. Scaled solar models and isochrones
We present a large and updated stellar evolution database for low-,
intermediate- and high-mass stars in a wide metallicity range, suitable for
studying Galactic and extragalactic simple and composite stellar populations
using population synthesis techniques. The stellar mass range is between
\sim0.5Mo and 10Mo with a fine mass spacing. The metallicity [Fe/H] comprises
10 values ranging from -2.27 to 0.40, with a scaled solar metal distribution.
The initial He mass fraction ranges from Y=0.245, for the more metal-poor
composition, up to 0.303 for the more metal-rich one, with Delta Y/Delta Z\sim
1.4. For each adopted chemical composition, the evolutionary models have been
computed without and with overshooting from the Schwarzschild boundary of the
convective cores during the central H-burning phase. The whole set of
evolutionary models can be used to compute isochrones in a wide age range, from
\sim30 Myr to \sim15Gyr. Both evolutionary tracks and isochrones are available
in several observational planes, employing updated set of bolometric
corrections and color-Te relations computed for this project. The number of
points along the models and the resulting isochrones is selected in such a way
that interpolation for intermediate metallicities not contained in the grid is
straightforward. We compare our isochrones with results from different stellar
evolution databases and perform some empirical tests for the reliability of our
models. Since this work is devoted to scaled solar compositions, we focus our
attention on the Galactic disk populations, employing multicolor photometry of
unevolved field MS stars with precise Hipparcos parallaxes, well-studied open
clusters and one eclipsing binary system with precise measurements of masses,
radii and [Fe/H] of both components.Comment: 57 pages, 26 figures (figs. 3, 4, 6 are at low resolution), ApJ in
press, the whole set of evolutionary results can be retrieved at
http://www.te.astro.it/BASTI/index.ph