6,592 research outputs found
New optical and near-infrared Surface Brightness Fluctuations models. A primary distance indicator ranging from Globular Clusters to distant galaxies?
We present new theoretical models for Surface Brightness Fluctuations (SBF)
both for optical and near-infrared bands in standard ground-based and Hubble
Space Telescope filter systems. Simple Stellar Population simulations are
adopted. Models cover the age and metallicity ranges from to and
from to 0.04 respectively. Effects due to the variation of the
Initial Mass Function and the stellar color-temperature relations are explored.
Particular attention is devoted to very bright stars in the color-magnitude
diagram and to investigate the effects of mass loss along the Red Giant Branch
(RGB) and the Asymptotic Giant Branch (AGB). It is found that and bands
SBF amplitudes are powerful diagnostics for the morphology of the Horizontal
Branch and the Post-AGB stars population. We point out that a careful treatment
of mass loss process along the RGB and AGB is fundamental in determining
reliable SBF evaluations. The SBF measurements are used to give robust
constraints on the evolution of AGB stars, suggesting that mass loss activity
on AGB stars should be twice more efficient than on the RGB stars. Our models
are able to reproduce the absolute SBF magnitudes of the Galactic Globular
Clusters and of galaxies, and their integrated colors. New calibrations of
absolute SBF magnitude in , , , and photometric filters are
provided, which appear reliable enough to directly gauge distances bypassing
other distance indicators. The SBF technique is also used as stellar population
tracer to derive age and metallicity of a selected sample of galaxies of known
distances. Finally, {\it SBF color} versus {\it integrated color} diagrams are
proposed as particularly useful in removing the well known {\it age-metallicity
degeneracy} affecting our knowledge of remote stellar systems.Comment: AJ accepted, 46 pages, 21 figures, 10 tables, uses aastex.cl
Disk Galaxy Formation in a LambdaCDM Universe
We describe hydrodynamical simulations of galaxy formation in a Lambda cold
dark matter (CDM) cosmology performed using a subresolution model for star
formation and feedback in a multiphase interstellar medium (ISM). In
particular, we demonstrate the formation of a well-resolved disk galaxy. The
surface brightness profile of the galaxy is exponential, with a B-band central
surface brightness of 21.0 mag arcsec^-2 and a scale-length of R_d = 2.0 h^-1
kpc. We find no evidence for a significant bulge component. The simulated
galaxy falls within the I-band Tully-Fisher relation, with an absolute
magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot=121.3 km
s^-1. While the total specific angular momentum of the stars in the galaxy
agrees with observations, the angular momentum in the inner regions appears to
be low by a factor of ~2. The star formation rate of the galaxy peaks at ~7
M_sun yr^-1 between redshifts z=2-4, with the mean stellar age decreasing from
\~10 Gyrs in the outer regions of the disk to ~7.5 Gyrs in the center,
indicating that the disk did not simply form inside-out. The stars exhibit a
metallicity gradient from 0.7 Z_sun at the edge of the disk to 1.3 Z_sun in the
center. Using a suite of idealized galaxy formation simulations with different
models for the ISM, we show that the effective pressure support provided by
star formation and feedback in our multiphase model is instrumental in allowing
the formation of large, stable disk galaxies. If ISM gas is instead modeled
with an isothermal equation of state, or if star formation is suppressed
entirely, growing gaseous disks quickly violate the Toomre stability criterion
and undergo catastrophic fragmentation.Comment: 14 pages, 12 figures, LaTex (emulateapj.cls), submitted to ApJ, high
resolution images available at
http://www-cfa.harvard.edu/~brobertson/papers/galaxy
Spectroscopy and Time Variability of Absorption Lines in the Direction of the Vela Supernova Remnant
We present high resolution (R~75,000), high signal-to-noise (S/N~100) Ca II
3933.663 and Na I 5889.951, 5895.924 spectra of 68
stars in the direction of the Vela supernova remnant. The spectra comprise the
most complete high resolution, high S/N, optical survey of early type stars in
this region of the sky. A subset of the sight lines has been observed at
multiple epochs, 1993/1994 and 1996. Of the thirteen stars observed twice,
seven have spectra revealing changes in the equivalent width and/or velocity
structure of lines, most of which arise from remnant gas. Such time variability
has been reported previously for the sight lines towards HD 72089 and HD 72997
by Danks & Sembach (1995) and for HD 72127 by Hobbs et al. (1991). We have
confirmed the ongoing time variability of these spectra and present new
evidence of variability in the spectra of HD 73658, HD 74455, HD 75309 and HD
75821. We have tabulated Na I and Ca II absorption line information for the
sight lines in our sample to serve as a benchmark for further investigations of
the dynamics and evolution of the Vela SNR.Comment: 8 pages of text, 4 tables, 16 pages of figures Accepted and to be
published in ApJ
The Distance of the Gamma-ray Binary 1FGL J1018.6-5856
The recently discovered gamma-ray binary 1FGL J1018.6-5856 has a proposed
optical/near-infrared (OIR) counterpart 2MASS 10185560-5856459. We present
Stromgren photometry of this star to investigate its photometric variability
and measure the reddening and distance to the system. We find that the
gamma-ray binary has E(B-V) = 1.34 +/- 0.04 and d = 5.4^+4.6_-2.1 kpc. While
E(B-V) is consistent with X-ray observations of the neutral hydrogen column
density, the distance is somewhat closer than some previous authors have
suggested.Comment: Accepted to PAS
A delta Scuti distance to the Large Magellanic Cloud
We present results from a well studied delta Scuti star discovered in the
LMC. The absolute magnitude of the variable was determined from the PL relation
for Galactic delta Scuti stars and from the theoretical modeling of the
observed B,V,I light curves. The two methods give distance moduli for the LMC
of 18.46+-0.19 and 18.48+-0.15, respectively, for a consistent value of the
stellar reddening of E(B-V)=0.08+-0.02. We have also analyzed 24 delta Scuti
candidates discovered in the OGLE II survey of the LMC, and 7 variables
identified in the open cluster LW 55 and in the galaxy disk by Kaluzny et al.
(2003, 2006). We find that the LMC delta Scuti stars define a PL relation whose
slope is very similar to that defined by the Galactic delta Scuti variables,
and yield a distance modulus for the LMC of 18.50+-0.22 mag. We compare the
results obtained from the delta Scuti variables with those derived from the LMC
RR Lyrae stars and Cepheids. Within the observational uncertainties, the three
groups of pulsating stars yield very similar distance moduli. These moduli are
all consistent with the "long" astronomical distance scale for the Large
Magellanic Cloud.Comment: Accepted for publication on A
The Palomar Testbed Interferometer Calibrator Catalog
The Palomar Testbed Interferometer (PTI) archive of observations between 1998
and 2005 is examined for objects appropriate for calibration of optical
long-baseline interferometer observations - stars that are predictably
point-like and single. Approximately 1,400 nights of data on 1,800 objects were
examined for this investigation. We compare those observations to an
intensively studied object that is a suitable calibrator, HD217014, and
statistically compare each candidate calibrator to that object by computing
both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis
is that the frequency distribution of visibility data associated with
calibrator stars differs from non-calibrator stars such as binary stars.
Spectroscopic binaries resolved by PTI, objects known to be unsuitable for
calibrator use, are similarly tested to establish detection limits of this
approach. From this investigation, we find more than 350 observed stars
suitable for use as calibrators (with an additional being
rejected), corresponding to sky coverage for PTI. This approach
is noteworthy in that it rigorously establishes calibration sources through a
traceable, empirical methodology, leveraging the predictions of spectral energy
distribution modeling but also verifying it with the rich body of PTI's on-sky
observations.Comment: 100 pages, 7 figures, 7 tables; to appear in the May 2008ApJS, v176n
Photometric Properties of Kiso Ultraviolet-Excess Galaxies in the Lynx-Ursa Major Region
We have performed a systematic study of several regions in the sky where the
number of galaxies exhibiting star formation (SF) activity is greater than
average. We used Kiso ultraviolet-excess galaxies (KUGs) as our SF-enhanced
sample. By statistically comparing the KUG and non-KUG distributions, we
discovered four KUG-rich regions with a size of . One of these regions corresponds spatially to a filament of length
Mpc in the Lynx-Ursa Major region (). We call this ``the Lynx-Ursa
Major (LUM) filament''. We obtained surface photometry of 11 of
the KUGs in the LUM filament and used these to investigate the integrated
colors, distribution of SF regions, morphologies, and local environments. We
found that these KUGs consist of distorted spiral galaxies and compact galaxies
with blue colors. Their star formation occurs in the entire disk, and is not
confined to just the central regions. The colors of the SF regions imply that
active star formation in the spiral galaxies occurred yr ago,
while that of the compact objects occurred yr ago. Though the
photometric characteristics of these KUGs are similar to those of interacting
galaxies or mergers, most of these KUGs do not show direct evidence of merger
processes.Comment: 39 pages LaTeX, using aasms4.sty, 20 figures, ApJS accepted. The
Title of the previous one was truncated by the author's mistake, and is
corrected. Main body of the paper is unchange
Gamma Doradus Stars: Defining a New Class of Pulsating Variables
In this paper we describe a new class of pulsating stars, the prototype of
which is the bright, early, F-type dwarf, Gamma Doradus. These stars typically
have between 1 and 5 periods ranging from 0.4 to 3 days with photometric
amplitudes up to 0.1 in Johnson V. The mechanism for these observed variations
is high-order, low-degree, non-radial, gravity-mode pulsation. Gamma Doradus
stars exhibit variability on a time scale that is an order of magnitude slower
than Delta Scuti stars. They may offer additional insight into stellar physics
when they are better understood (e.g., they may represent the cool portion of
an "iron opacity instability strip" currently formed by the Beta Cephei stars,
the SPB stars, and the subdwarf B stars; they may also offer insight into the
presence of g-modes in solar-like stars).Comment: 15 pages, 1 ps figure. Accepted for publication in PAS
The in-process control of PVC sheath of a double core cable
In this work the possibility of the sheath hermiticity testing by measuring of the cable capacity per unit length variation during spark testing is considered. The research object is 2•0.75 HO3VVH2-F cable. According to the physical modelling it is proved that such defect of sheath as pinhole through the whole thickness of sheath can be registered for the test length 10 cm with test voltage frequencies 1kHz and 10kHz
Si nanoparticle interfaces in Si/SiO2 solar cell materials
Novel solar cell materials consisting of Si nanoparticles embedded in SiO2 layers have been studied using positron annihilation spectroscopy in Doppler broadening mode and photoluminescence. Two positron-trapping interface states are observed after high temperature annealing at 1100 °C. One of the states is attributed to the (SiO2/Si bulk) interface and the other to the interface between the Si nanoparticles and SiO2. A small reduction in positron trapping into these states is observed after annealing the samples in N2 atmosphere with 5% H2. Enhanced photoluminescence is also observed from the samples following this annealing step.Peer reviewe
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