8,571 research outputs found
Two New Tests of the Metallicity Sensitivity of the Cepheid Period-Luminosity Relation (The Leavitt Law)
We undertake a new test of the metallicity sensitivity of the Leavitt Law for
Classical Cepheids. We derive an empirical calibration of the apparent
luminosities of Cepheids as measured from the optical through the mid-infrared
(0.45-8.0um) as a function of spectroscopic [Fe/H] abundances of individual
Cepheids in the Large Magellanic Cloud from Romaniello et al. (2008). The
cumulative trend over the entire wavelength range shows a nearly monotonic
behavior. The sense of the trend is consistent with differential
line-blanketing in the optical, leading to stars of high metallicity being
fainter in the optical. This is followed by a reversal in the trend at longer
wavelengths, with the cross-over occurring near the K band at about 2.2um,
consistent with a subsequent redistribution of energy resulting in a mild
brightening of Cepheids (with increased metallicity) at mid-infrared
wavelengths. This conclusion agrees with that of Romaniello et al. based on a
differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy,
SMC and LMC, but is opposite in sign to most other empirical tests of the
sensitivity of Cepheid distances to mean [O/H] HII region abundances. We also
search for a correlation of Cepheid host-galaxy metallicity with deviations of
the galaxy's Cepheid distance from that predicted from a pure Hubble flow.
Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very
weak signal is detected giving Dmu_o = -0.17 (+/- 0.31) ([O/H] - 8.80) - 0.21
(+/-0.10). This is in agreement with previous determinations, but statistically
inconclusive.Comment: Accepted to the Astrophysical Journal. 8 pages, 5 figures, 1 tabl
Analysis of RR Lyrae Stars in the Northern Sky Variability Survey
We use data from the Northern Sky Variability Survey (NSVS), obtained from
the first generation Robotic Optical Transient Search Experiment (ROTSE-I), to
identify and study RR Lyrae variable stars in the solar neighborhood. We
initially identified 1197 RRab (RR0) candidate stars brighter than the ROTSE
median magnitude V = 14. Periods, amplitudes, and mean V magnitudes are
determined for a subset of 1188 RRab stars with well defined light curves.
Metallicities are determined for 589 stars by the Fourier parameter method and
by the relationship between period, amplitude, and [Fe/H]. We comment upon the
difficulties of clearly classifying RRc (RR1) variables in the NSVS dataset.
Distances to the RRab stars are calculated using an adopted
luminosity-metallicity relation with corrections for interstellar extinction.
The 589 RRab stars in our final sample are used to study the properties of the
RRab population within 5 kpc of the Sun. The Bailey diagram of period versus
amplitude shows that the largest component of this sample belongs to Oosterhoff
type I. Metal-rich ([Fe/H] > -1) RRab stars appear to be associated with the
Galactic disk. Our metal-rich RRab sample may include a thin disk as well as a
thick disk population, although the uncertainties are too large to establish
this. There is some evidence among the metal-rich RRab stars for a decline in
scale height with increasing [Fe/H], as was found by Layden (1995). The
distribution of RRab stars with -1 < [Fe/H] < -1.25 indicates that within this
metallicity range the RRab stars are a mixture of stars belonging to halo and
disk populations.Comment: 68 pages, 26 figures, 9 tables, accepted to A
Detection of a Temperature Inversion in the Broadband Infrared Emission Spectrum of TrES-4
We estimate the strength of the bandpass-integrated thermal emission from the
extrasolar planet TrES-4 at 3.6, 4.5, 5.8, and 8.0 micron using the Infrared
Array Camera (IRAC) on the Spitzer Space Telescope. We find relative eclipse
depths of 0.137 +/- 0.011%, 0.148 +/- 0.016%, 0.261 +/- 0.059%, and 0.318 +/-
0.044% in these four bandpasses, respectively. We also place a 2 sigma upper
limit of 0.37% on the depth of the secondary eclipse in the 16 micron IRS
peak-up array. These eclipse depths reveal that TrES-4 has an emission spectrum
similar to that of HD 209458b, which requires the presence of water emission
bands created by an thermal inversion layer high in the atmosphere in order to
explain the observed features. TrES-4 receives more radiation from its star
than HD 209458b and has a correspondingly higher effective temperature,
therefore the presence of a temperature inversion in this planet's atmosphere
lends support to the idea that inversions might be correlated with the
irradiance received by the planet. We find no evidence for any offset in the
timing of the secondary eclipse, and place a 3 sigma upper limit of
|ecos(omega)|<0.0058 where e is the planet's orbital eccentricity and omega is
the argument of pericenter. From this we conclude that tidal heating from
ongoing orbital circulatization is unlikely to be the explanation for TrES-4's
inflated radius.Comment: 10 pages in emulateapj format, 7 figures (some in color), accepted
for publication in Ap
A Calibration of NICMOS Camera 2 for Low Count-Rates
NICMOS 2 observations are crucial for constraining distances to most of the
existing sample of z > 1 SNe Ia. Unlike the conventional calibration programs,
these observations involve long exposure times and low count rates. Reciprocity
failure is known to exist in HgCdTe devices and a correction for this effect
has already been implemented for high and medium count-rates. However
observations at faint count-rates rely on extrapolations. Here instead, we
provide a new zeropoint calibration directly applicable to faint sources. This
is obtained via inter-calibration of NIC2 F110W/F160W with WFC3 in the low
count-rate regime using z ~ 1 elliptical galaxies as tertiary calibrators.
These objects have relatively simple near-IR SEDs, uniform colors, and their
extended nature gives superior signal-to-noise at the same count rate than
would stars. The use of extended objects also allows greater tolerances on PSF
profiles. We find ST magnitude zeropoints (after the installation of the NICMOS
cooling system, NCS) of 25.296 +- 0.022 for F110W and 25.803 +- 0.023 for
F160W, both in agreement with the calibration extrapolated from count-rates
1,000 times larger (25.262 and 25.799). Before the installation of the NCS, we
find 24.843 +- 0.025 for F110W and 25.498 +- 0.021 for F160W, also in agreement
with the high-count-rate calibration (24.815 and 25.470). We also check the
standard bandpasses of WFC3 and NICMOS 2 using a range of stars and galaxies at
different colors and find mild tension for WFC3, limiting the accuracy of the
zeropoints. To avoid human bias, our cross-calibration was "blinded" in that
the fitted zeropoint differences were hidden until the analysis was finalized.Comment: Accepted for Publication in the Astronomical Journal. New version
contains added referenc
Discovery of Two Distant Type Ia Supernovae in the Hubble Deep Field North with the Advanced Camera for Surveys
We present observations of the first two supernovae discovered with the
recently installed Advanced Camera for Surveys (ACS) on the Hubble Space
Telescope. The supernovae were found in Wide Field Camera images of the Hubble
Deep Field North taken with the F775W, F850LP, and G800L optical elements as
part of the ACS guaranteed time observation program. Spectra extracted from the
ACS G800L grism exposures confirm that the objects are Type Ia supernovae (SNe
Ia) at redshifts z=0.47 and z=0.95. Follow-up HST observations have been
conducted with ACS in F775W and F850LP and with NICMOS in the near-infrared
F110W bandpass, yielding a total of 9 flux measurements in the 3 bandpasses
over a period of 50 days in the observed frame. We discuss many of the
important issues in doing accurate photometry with the ACS. We analyze the
multi-band light curves using two different fitting methods to calibrate the
supernovae luminosities and place them on the SNe Ia Hubble diagram. The
resulting distances are consistent with the redshift-distance relation of the
accelerating universe model, although evolving intergalactic grey dust remains
as a less likely possibility. The relative ease with which these SNe Ia were
found, confirmed, and monitored demonstrates the potential ACS holds for
revolutionizing the field of high-redshift SNe Ia, and therefore of testing the
accelerating universe cosmology and constraining the "epoch of deceleration".Comment: 11 pages, 8 embedded figures. Accepted for publication in Ap
Supernova Constraints and Systematic Uncertainties from the First Three Years of the Supernova Legacy Survey
We combine high-redshift Type Ia supernovae from the first three years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-z, 93 SDSS, 242 SNLS, and 14 Hubble Space Telescope). SN data alone require cosmic acceleration at >99.999% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least z = 1.4) in a flat universe, we find w = –0.91^(+0.16)_(–0.20)(stat)^(+0.07)_(–0.14)(sys) from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between host-galaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematic covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems
Kepler Input Catalog: Photometric Calibration and Stellar Classification
We describe the photometric calibration and stellar classification methods
used to produce the Kepler Input Catalog (KIC). The KIC is a catalog containing
photometric and physical data for sources in the Kepler Mission field of view;
it is used by the mission to select optimal targets. We derived atmospheric
extinction corrections from hourly observations of secondary standard fields
within the Kepler field of view. Repeatability of absolute photometry for stars
brighter than magnitude 15 is typically 2%. We estimated stellar parameters
Teff, log(g), log (Z), E_{B-V} using Bayesian posterior probability
maximization to match observed colors to Castelli stellar atmosphere models. We
applied Bayesian priors describing the distribution of solar-neighborhood stars
in the color-magnitude diagram (CMD), in log (Z)$, and in height above the
galactic plane. Comparisons with samples of stars classified by other means
indicate that in most regions of the CMD, our classifications are reliable
within about +/- 200 K and +/- 0.4 dex in log (g). It is difficult to assess
the reliability of our log(Z) estimates, but there is reason to suspect that it
is poor, particularly at extreme Teff. Of great importance for the Kepler
Mission, for Teff <= 5400 K, the distinction between main-sequence stars and
giants has proved to be reliable with better than 98% confidence. The KIC is
available through the MAST data archive.Comment: 77 pages, 12 figures, 1 Table. Accepted by Astronomical Journal 24
July 201
Searching for Planets in the Hyades II: Some Implications of Stellar Magnetic Activity
The Hyades constitute a homogeneous sample of stars ideal for investigating
the dependence of planet formation on the mass of the central star. Due to
their youth, Hyades members are much more chromospherically active than stars
traditionally surveyed for planets using high precision radial velocity (RV)
techniques. Therefore, we have conducted a detailed investigation of whether
magnetic activity of our Hyades target stars will interfere with our ability to
make precise RV searches for substellar companions. We measure chromospheric
activity (which we take as a proxy for magnetic activity) by computing the
equivalent of the R'HK activity index from the Ca II K line. is not
constant in the Hyades: we confirm that it decreases with increasing
temperature in the F stars, and also find it decreases for stars cooler than
mid-K. We examine correlations between simultaneously measured R'HK and RV
using both a classical statistical test and a Bayesian odds ratio test. We find
that there is a significant correlation between R'HK and the RV in only 5 of
the 82 stars in this sample. Thus, simple Rprime HK-RV correlations will
generally not be effective in correcting the measured RV values for the effects
of magnetic activity in the Hyades. We argue that this implies long timescale
activity variations (of order a few years; i.e., magnetic cycles or growth and
decay of plage regions) will not significantly hinder our search for planets in
the Hyades if the stars are closely monitored for chromospheric activity. The
trends in the RV scatter (sigma'_v) with , vsini, and P_rot for our stars
is generally consistent with those found in field stars in the Lick planet
search data, with the notable exception of a shallower dependence of sigma'_v
on for F stars.Comment: 15 pages, 7 figures, 3 tables; To appear in the July 2002 issue of
The Astronomical Journa
A Systematic Search for New Coupling Schemes of Cross-Coupled Resonator Bandpass Filters
In this paper, a systematic approach to an extensive search for topologies of cross-coupled filters with generalized Chebyshev response is presented. The technique applies graph theory to find unique, nonisomorphic filter configurations, and tests whether a specific frequency response can be realized in a given set of topologies. The results of the search are then stored in a database of possible filter configurations
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