117 research outputs found
Cool Customers in the Stellar Graveyard I: Limits to Extrasolar Planets Around the White Dwarf G29-38
We present high contrast images of the hydrogen white dwarf G 29-38 taken in
the near infrared with the Hubble Space Telescope and the Gemini North
Telescope as part of a high contrast imaging search for substellar objects in
orbit around nearby white dwarfs.
We review the current limits on planetary companions for G29-38, the only
nearby white dwarf with an infrared excess due to a dust disk. We add our
recent observations to these limits to produce extremely tight constraints on
the types of possible companions that could be present. No objects 6
M are detected in our data at projected separations 12 AU, and no
objects 16 M are detected for separations from 3 to 12 AU, assuming
a total system age of 1 Gyr. Limits for companions at separations 3 AU come
from a combination of 2MASS photometry and previous studies of G29-38's
pulsations. Our imaging with Gemini cannot confirm a tentative claim for the
presence of a low mass brown dwarf. These observations demonstrate that a
careful combination of several techniques can probe nearby white dwarfs for
large planets and low mass brown dwarfs.Comment: 20 pages, 4 figures, Accepted to Ap
A Comprehensive Spectroscopic Analysis of DB White Dwarfs
We present a detailed analysis of 108 helium-line (DB) white dwarfs based on
model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a
mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White
dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample,
and their mass distribution appears similar to that of DB stars. As in our
previous investigation, we find no evidence for the existence of low-mass (M <
0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of
DB white dwarfs identified in the Palomar-Green survey. We show that 20% of all
white dwarfs in the temperature range of interest are DB stars, although the
fraction drops to half this value above Teff ~ 20,000 K. We also show that the
persistence of DB stars with no hydrogen features at low temperatures is
difficult to reconcile with a scenario involving accretion from the
interstellar medium, often invoked to account for the observed hydrogen
abundances in DBA stars. We present evidence for the existence of two different
evolutionary channels that produce DB white dwarfs: the standard model where DA
stars are transformed into DB stars through the convective dilution of a thin
hydrogen layer, and a second channel where DB stars retain a helium-atmosphere
throughout their evolution. We finally demonstrate that the instability strip
of pulsating V777 Her white dwarfs contains no nonvariables, if the hydrogen
content of these stars is properly accounted for.Comment: 74 pages including 30 figures, accepted for publication in the
Astrophysical Journa
NLTE model calculations for the solar atmosphere with an iterative treatment of opacity distribution functions
Modeling the variability of the solar spectral irradiance is a key factor for
understanding the solar influence on the climate of the Earth. As a first step
to calculating the solar spectral irradiance variations we reproduce the solar
spectrum for the quiet Sun over a broad wavelength range with an emphasis on
the UV. We introduce the radiative transfer code COSI which calculates solar
synthetic spectra under conditions of non-local thermodynamic equilibrium
(NLTE). A self-consistent simultaneous solution of the radiative transfer and
the statistical equation for the level populations guarantees that the correct
physics is considered for wavelength regions where the assumption of local
thermodynamic equilibrium (LTE) breaks down. The new concept of iterated
opacity distribution functions (NLTE-ODFs), through which all line opacities
are included in the NLTE radiative transfer calculation, is presented. We show
that it is essential to include the line opacities in the radiative transfer to
reproduce the solar spectrum in the UV. Through the implemented scheme of
NLTE-ODFs the COSI code is successful in reproducing the spectral energy
distribution of the quiet Sun.Comment: 13 pages, 9 figures. accepted for publication in Astronomy and
Astrophysic
Transiting Disintegrating Planetary Debris around WD 1145+017
More than a decade after astronomers realized that disrupted planetary
material likely pollutes the surfaces of many white dwarf stars, the discovery
of transiting debris orbiting the white dwarf WD 1145+017 has opened the door
to new explorations of this process. We describe the observational evidence for
transiting planetary material and the current theoretical understanding (and in
some cases lack thereof) of the phenomenon.Comment: Invited review chapter. Accepted March 23, 2017 and published October
7, 2017 in the Handbook of Exoplanets. 15 pages, 10 figure
The Type Ia Supernova Rate in Redshift 0.5--0.9 Galaxy Clusters
Supernova (SN) rates are potentially powerful diagnostics of metal enrichment
and SN physics, particularly in galaxy clusters with their deep,
metal-retaining potentials and relatively simple star-formation histories. We
have carried out a survey for supernovae (SNe) in galaxy clusters, at a
redshift range 0.5<z<0.9, using the Advanced Camera for Surveys (ACS) on the
Hubble Space Telescope. We reimaged a sample of 15 clusters that were
previously imaged by ACS, thus obtaining two to three epochs per cluster, in
which we discovered five likely cluster SNe, six possible cluster SNe Ia, two
hostless SN candidates, and several background and foreground events. Keck
spectra of the host galaxies were obtained to establish cluster membership. We
conducted detailed efficiency simulations, and measured the stellar
luminosities of the clusters using Subaru images. We derive a cluster SN rate
of 0.35 SNuB +0.17/-0.12 (statistical) \pm0.13 (classification) \pm0.01
(systematic) [where SNuB = SNe (100 yr 10^10 L_B_sun)^-1] and 0.112 SNuM
+0.055/-0.039 (statistical) \pm0.042 (classification) \pm0.005 (systematic)
[where SNuM = SNe (100 yr 10^10 M_sun)^-1]. As in previous measurements of
cluster SN rates, the uncertainties are dominated by small-number statistics.
The SN rate in this redshift bin is consistent with the SN rate in clusters at
lower redshifts (to within the uncertainties), and shows that there is, at
most, only a slight increase of cluster SN rate with increasing redshift. The
low and fairly constant SN Ia rate out to z~1 implies that the bulk of the iron
mass in clusters was already in place by z~1. The recently observed doubling of
iron abundances in the intracluster medium between z=1 and 0, if real, is
likely the result of redistribution of existing iron, rather than new
production of iron.Comment: Accepted to ApJ. Full resolution version available at
http://kicp.uchicago.edu/~kerens/HSTclusterSNe
Rotation and activity in the solar-metallicity open cluster NGC2516
We report new measures of radial velocities and rotation rates (v sin i) for
51 F and early-G stars in the open cluster NGC2516, and combine these with
previously published data. From high signal-to-noise spectra of two stars, we
show that NGC2516 has a relative iron abundance with respect to the Pleiades of
delta([Fe/H])= +0.04 +/- 0.07 at the canonical reddening of E(B - V) = 0.12, in
contrast to previous photometric studies that placed the cluster 0.2 to 0.4 dex
below solar. We construct a color-magnitude diagram based on radial velocity
members, and explore the sensitivity of photometric determinations of the
metallicity and distance to assumed values of the reddening. For a metal
abundance near solar, the Hipparcos distance to NGC2516 is probably
underestimated. Finally, we show that the distribution of rotation rates and
X-ray emission does not differ greatly from that of the Pleiades, when
allowance is made for the somewhat older age of NGC2516.Comment: Accepted for publication in the Astrophysical Journal. 35 pages
including 7 figure
The COS-Halos Survey: Keck LRIS and Magellan MagE Optical Spectroscopy
We present high signal-to-noise optical spectra for 67 low-redshift (0.1 < z
< 0.4) galaxies that lie within close projected distances (5 kpc < rho < 150
kpc) of 38 background UV-bright QSOs. The Keck LRIS and Magellan MagE data
presented here are part of a survey that aims to construct a statistically
sampled map of the physical state and metallicity of gaseous galaxy halos using
the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). We
provide a detailed description of the optical data reduction and subsequent
spectral analysis that allow us to derive the physical properties of this
uniquely data-rich sample of galaxies. The galaxy sample is divided into 38
pre-selected L ~ L*, z ~ 0.2 "target" galaxies and 29 "bonus" galaxies that lie
in close proximity to the QSO sightlines. We report galaxy spectroscopic
redshifts accurate to +/- 30 km s-1, impact parameters, rest-frame colors,
stellar masses, total star formation rates, and gas-phase interstellar medium
oxygen abundances. When we compare the distribution of these galaxy
characteristics to those of the general low-redshift population, we find good
agreement. The L ~ L* galaxies in this sample span a diverse range of color
(1.0 < u-r < 3.0), stellar mass (10^9.5 < M/M_sun < 10^11.5), and SFRs (0.01 -
20 M_sun yr-1). These optical data, along with the COS UV spectroscopy,
comprise the backbone of our efforts to understand how halo gas properties may
correlate with their host galaxy properties, and ultimately to uncover the
processes that drive gas outflow and/or are influenced by gas inflow.Comment: 20 pages, 12 Figures, Submitted to ApJ
A Spectroscopic Survey & Analysis of Bright, Hydrogen-Rich White Dwarfs
We have conducted a spectroscopic survey of over 1300 bright (V < 17.5),
hydrogen-rich white dwarfs based largely on the last published version of the
McCook & Sion catalog. The complete results from our survey, including the
spectroscopic analysis of over 1100 DA white dwarfs, are presented. High
signal-to-noise ratio optical spectra were obtained for each star and were
subsequently analyzed using our standard spectroscopic technique where the
observed Balmer line profiles are compared to synthetic spectra computed from
the latest generation of model atmospheres appropriate for these stars. First,
we present the spectroscopic content of our sample, which includes many
misclassifications as well as several DAB, DAZ, and magnetic white dwarfs.
Next, we look at how the new Stark broadening profiles affect the determination
of the atmospheric parameters. When necessary, specific models and analysis
techniques are used to derive the most accurate atmospheric parameters
possible. In particular, we employ M dwarf templates to obtain better estimates
of the atmospheric parameters for those white dwarfs that are in DA+dM binary
systems. Certain unique white dwarfs and double-degenerate binary systems are
also analyzed in greater detail. We then examine the global properties of our
sample including the mass distribution and their distribution as a function of
temperature. We then proceed to test the accuracy and robustness of our method
by comparing our results to those of other surveys such as SPY and SDSS.
Finally, we revisit the ZZ Ceti instability strip and examine how the
determination of its empirical boundaries are affected by the latest line
profile calculations.Comment: 32 pages, 35 figures, 5 tables, accepted for publication in the
Astrophysical Journa
The Atacama Cosmology Telescope: Cosmology from Galaxy Clusters Detected via the Sunyaev-Zel'dovich Effect
We present constraints on cosmological parameters based on a sample of
Sunyaev-Zel'dovich-selected galaxy clusters detected in a millimeter-wave
survey by the Atacama Cosmology Telescope. The cluster sample used in this
analysis consists of 9 optically-confirmed high-mass clusters comprising the
high-significance end of the total cluster sample identified in 455 square
degrees of sky surveyed during 2008 at 148 GHz. We focus on the most massive
systems to reduce the degeneracy between unknown cluster astrophysics and
cosmology derived from SZ surveys. We describe the scaling relation between
cluster mass and SZ signal with a 4-parameter fit. Marginalizing over the
values of the parameters in this fit with conservative priors gives sigma_8 =
0.851 +/- 0.115 and w = -1.14 +/- 0.35 for a spatially-flat wCDM cosmological
model with WMAP 7-year priors on cosmological parameters. This gives a modest
improvement in statistical uncertainty over WMAP 7-year constraints alone.
Fixing the scaling relation between cluster mass and SZ signal to a fiducial
relation obtained from numerical simulations and calibrated by X-ray
observations, we find sigma_8 = 0.821 +/- 0.044 and w = -1.05 +/- 0.20. These
results are consistent with constraints from WMAP 7 plus baryon acoustic
oscillations plus type Ia supernoava which give sigma_8 = 0.802 +/- 0.038 and w
= -0.98 +/- 0.053. A stacking analysis of the clusters in this sample compared
to clusters simulated assuming the fiducial model also shows good agreement.
These results suggest that, given the sample of clusters used here, both the
astrophysics of massive clusters and the cosmological parameters derived from
them are broadly consistent with current models.Comment: 12 pages, 7 figures. Submitted to Ap
Utilizing international networks for accelerating research and learning in transformational sustainability science
A promising approach for addressing sustainability problems is to recognize the unique conditions of a particular place, such as problem features and solution capabilities, and adopt and adapt solutions developed at other places around the world. Therefore, research and teaching in international networks becomes critical, as it allows for accelerating learning by sharing problem understandings, successful solutions, and important contextual considerations. This article identifies eight distinct types of research and teaching collaborations in international networks that can support such accelerated learning. The four research types are, with increasing intensity of collaboration: (1) solution adoption; (2) solution consultation; (3) joint research on different problems; and (4) joint research on similar problems. The four teaching types are, with increasing intensity of collaboration: (1) adopted course; (2) course with visiting faculty; (3) joint course with traveling faculty; and (4) joint course with traveling students. The typology is illustrated by extending existing research and teaching projects on urban sustainability in the International Network of Programs in Sustainability, with partner universities from Europe, North America, Asia, and Africa. The article concludes with challenges and strategies for extending individual projects into collaborations in international networks.Postprint (author's final draft
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