189 research outputs found
A cool, magnetic white dwarf accreting planetary debris
We present an analysis of spectroscopic data of the cool, highly magnetic and
polluted white dwarf 2MASS J0916-4215. The atmosphere of the white dwarf is
dominated by hydrogen, but numerous spectral lines of magnesium, calcium,
titanium, chromium, iron, strontium, along with Li I, Na I, Al I, and K I
lines, are found in the incomplete Paschen-Back regime, most visibly, in the
case of Ca II lines. Extensive new calculations of the Paschen-Back effect in
several spectral lines are presented and results of the calculations are
tabulated for the Ca II H&K doublet. The abundance pattern shows a large
lithium and strontium excess, which may be viewed as a signature of planetary
debris akin to Earth's continental crust accreted onto the star, although the
scarcity of silicon indicates possible dilution in bulk Earth material.
Accurate abundance measurements proved sensitive to the value of the broadening
parameter due to collisions with neutral hydrogen (Gamma_HI), particularly in
saturated lines such as the resonance lines of Ca I and Ca II. We found that
Gamma_HI if formulated with values from the literature could be overestimated
by a factor of 10 in most resonance lines.Comment: Accepted for publication in MNRA
Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44
Ultraviolet and optical spectra of the hydrogen-dominated atmosphere white
dwarf star G238-44 obtained with FUSE, Keck/HIRES, HST/COS, and HST/STIS reveal
ten elements heavier than helium: C, N, O, Mg, Al, Si, P, S, Ca, and Fe.
G238-44 is only the third white dwarf with nitrogen detected in its atmosphere
from polluting planetary system material. Keck/HIRES data taken on eleven
nights over 24 years show no evidence for variation in the equivalent width of
measured absorption lines, suggesting stable and continuous accretion from a
circumstellar reservoir. From measured abundances and limits on other elements
we find an anomalous abundance pattern and evidence for the presence of
metallic iron. If the pollution is from a single parent body, then it would
have no known counterpart within the solar system. If we allow for two distinct
parent bodies, then we can reproduce the observed abundances with a mix of
iron-rich Mercury-like material and an analog of an icy Kuiper Belt object with
a respective mass ratio of 1.7:1. Such compositionally disparate objects would
provide chemical evidence for both rocky and icy bodies in an exoplanetary
system and would be indicative of a planetary system so strongly perturbed that
G238-44 is able to capture both asteroid- and Kuiper Belt-analog bodies
near-simultaneously within its 100 Myr cooling age.Comment: Accepted to ApJ. 18 pages, 9 figures, 7 table
Role of copper efflux in pneumococcal pathogenesis and resistance to macrophage-mediated immune clearance
In bacteria, the intracellular levels of metals are mediated by tightly controlled acquisition and efflux systems. This is particularly true of copper, a trace element that is universally toxic in excess. During infection, the toxic properties of copper are exploited by the mammalian host to facilitate bacterial clearance. To better understand the role of copper during infection, we characterized the contribution of the cop operon to copper homeostasis and virulence in Streptococcus pneumoniae. Deletion of either the exporter, encoded by copA, or the chaperone, encoded by cupA, led to hypersensitivity to copper stress. We further demonstrated that loss of the copper exporter encoded by copA led to decreased virulence in pulmonary, intraperitoneal, and intravenous models of infection. Deletion of copA resulted in enhanced macrophage-mediated bacterial clearance in vitro. The attenuation phenotype of the copA mutant in the lung was found to be dependent on pulmonary macrophages, underscoring the importance of copper efflux in evading immune defenses. Overall, these data provide insight into the role of the cop operon in pneumococcal pathogenesis
Disk or Companion: Characterizing Excess Infrared Flux in Seven White Dwarf Systems with Near-Infrared Spectroscopy
Excess infrared flux from white dwarf stars is likely to arise from a dusty
debris disk or a cool companion. In this work, we present near-infrared
spectroscopic observations with Keck/MOSFIRE, Gemini/GNIRS, and
Gemini/Flamingos-2 of seven white dwarfs with infrared excesses identified in
previous studies. We confirmed the presence of dust disks around four white
dwarfs (Gaia J0611-6931, Gaia J0006+2858, Gaia J2100+2122, and WD 0145+234) as
well as two new white dwarf brown dwarf pairs (Gaia J0052+4505 and Gaia
J0603+4518). In three of the dust disk systems, we detected for the first time
near-infrared metal emissions (Mg I, Fe I, and Si I) from a gaseous component
of the disk. We developed a new Markov Chain Monte Carlo framework to constrain
the geometric properties of each dust disk. In three systems, the dust disk and
the gas disk appear to coincide spatially. For the two brown dwarf white dwarf
pairs, we identified broad molecular absorption features typically seen in L
dwarfs. The origin of the infrared excess around Gaia J0723+6301 remains a
mystery. Our study underlines how near-infrared spectroscopy can be used to
determine sources of infrared excess around white dwarfs, which has now been
detected in hundreds of systems photometrically.Comment: 23 pages, 10 figures, 5 tables, AJ, in pres
Shallow Ultraviolet Transits of WD 1145+017
WD 1145+017 is a unique white dwarf system that has a heavily polluted
atmosphere, an infrared excess from a dust disk, numerous broad absorption
lines from circumstellar gas, and changing transit features, likely from
fragments of an actively disintegrating asteroid. Here, we present results from
a large photometric and spectroscopic campaign with Hubble, Keck , VLT,
Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat
surprisingly, but consistent with previous observations in the u' band, the UV
transit depths are always shallower than those in the optical. We develop a
model that can quantitatively explain the observed "bluing" and the main
findings are: I. the transiting objects, circumstellar gas, and white dwarf are
all aligned along our line of sight; II. the transiting object is blocking a
larger fraction of the circumstellar gas than of the white dwarf itself.
Because most circumstellar lines are concentrated in the UV, the UV flux
appears to be less blocked compared to the optical during a transit, leading to
a shallower UV transit. This scenario is further supported by the strong
anti-correlation between optical transit depth and circumstellar line strength.
We have yet to detect any wavelength-dependent transits caused by the
transiting material around WD 1145+017.Comment: 16 pages, 11 figures, 6 tables, ApJ, in pres
New chondritic bodies identified in eight oxygen-bearing white dwarfs
We present observations and analyses of eight white dwarf stars that have
accreted rocky material from their surrounding planetary systems. The spectra
of these helium-atmosphere white dwarfs contain detectable optical lines of all
four major rock-forming elements (O, Mg, Si, Fe). This work increases the
sample of oxygen-bearing white dwarfs with parent body composition analyses by
roughly thirty-three percent. To first order, the parent bodies that have been
accreted by the eight white dwarfs are similar to those of chondritic
meteorites in relative elemental abundances and oxidation states. Seventy-five
percent of the white dwarfs in this study have observed oxygen excesses
implying volatiles in the parent bodies with abundances similar to those of
chondritic meteorites. Three white dwarfs have oxidation states that imply more
reduced material than found in CI chondrites, indicating the possible detection
of Mercury-like parent bodies, but are less constrained. These results
contribute to the recurring conclusion that extrasolar rocky bodies closely
resemble those in our solar system, and do not, as a whole, yield unusual or
unique compositions.Comment: Accepted for publication in ApJ. 7 Figures, 7 Table
The Atacama Cosmology Telescope: Cosmological parameters from three seasons of data
We present constraints on cosmological and astrophysical parameters from
high-resolution microwave background maps at 148 GHz and 218 GHz made by the
Atacama Cosmology Telescope (ACT) in three seasons of observations from 2008 to
2010. A model of primary cosmological and secondary foreground parameters is
fit to the map power spectra and lensing deflection power spectrum, including
contributions from both the thermal Sunyaev-Zeldovich (tSZ) effect and the
kinematic Sunyaev-Zeldovich (kSZ) effect, Poisson and correlated anisotropy
from unresolved infrared sources, radio sources, and the correlation between
the tSZ effect and infrared sources. The power ell^2 C_ell/2pi of the thermal
SZ power spectrum at 148 GHz is measured to be 3.4 +\- 1.4 muK^2 at ell=3000,
while the corresponding amplitude of the kinematic SZ power spectrum has a 95%
confidence level upper limit of 8.6 muK^2. Combining ACT power spectra with the
WMAP 7-year temperature and polarization power spectra, we find excellent
consistency with the LCDM model. We constrain the number of effective
relativistic degrees of freedom in the early universe to be Neff=2.79 +\- 0.56,
in agreement with the canonical value of Neff=3.046 for three massless
neutrinos. We constrain the sum of the neutrino masses to be Sigma m_nu < 0.39
eV at 95% confidence when combining ACT and WMAP 7-year data with BAO and
Hubble constant measurements. We constrain the amount of primordial helium to
be Yp = 0.225 +\- 0.034, and measure no variation in the fine structure
constant alpha since recombination, with alpha/alpha0 = 1.004 +/- 0.005. We
also find no evidence for any running of the scalar spectral index, dns/dlnk =
-0.004 +\- 0.012.Comment: 26 pages, 22 figures. This paper is a companion to Das et al. (2013)
and Dunkley et al. (2013). Matches published JCAP versio
The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey
We report on extragalactic sources detected in a 455 square-degree map of the
southern sky made with data at a frequency of 148 GHz from the Atacama
Cosmology Telescope 2008 observing season. We provide a catalog of 157 sources
with flux densities spanning two orders of magnitude: from 15 to 1500 mJy.
Comparison to other catalogs shows that 98% of the ACT detections correspond to
sources detected at lower radio frequencies. Three of the sources appear to be
associated with the brightest cluster galaxies of low redshift X-ray selected
galaxy clusters. Estimates of the radio to mm-wave spectral indices and
differential counts of the sources further bolster the hypothesis that they are
nearly all radio sources, and that their emission is not dominated by
re-emission from warm dust. In a bright (>50 mJy) 148 GHz-selected sample with
complete cross-identifications from the Australia Telescope 20 GHz survey, we
observe an average steepening of the spectra between 5, 20, and 148 GHz with
median spectral indices of , , and . When the
measured spectral indices are taken into account, the 148 GHz differential
source counts are consistent with previous measurements at 30 GHz in the
context of a source count model dominated by radio sources. Extrapolating with
an appropriately rescaled model for the radio source counts, the Poisson
contribution to the spatial power spectrum from synchrotron-dominated sources
with flux density less than 20 mJy is C^{\rm Sync} = (2.8 \pm 0.3) \times
10^{-6} \micro\kelvin^2.Comment: Accepted to Ap
The Atacama Cosmology Telescope: A Measurement of the Cosmic Microwave Background Power Spectrum at 148 and 218 GHz from the 2008 Southern Survey
We present measurements of the cosmic microwave background (CMB) power
spectrum made by the Atacama Cosmology Telescope at 148 GHz and 218 GHz, as
well as the cross-frequency spectrum between the two channels. Our results
clearly show the second through the seventh acoustic peaks in the CMB power
spectrum. The measurements of these higher-order peaks provide an additional
test of the {\Lambda}CDM cosmological model. At l > 3000, we detect power in
excess of the primary anisotropy spectrum of the CMB. At lower multipoles 500 <
l < 3000, we find evidence for gravitational lensing of the CMB in the power
spectrum at the 2.8{\sigma} level. We also detect a low level of Galactic dust
in our maps, which demonstrates that we can recover known faint, diffuse
signals.Comment: 19 pages, 13 figures. Submitted to ApJ. This paper is a companion to
Hajian et al. (2010) and Dunkley et al. (2010
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