506 research outputs found
Two candidate brown dwarf companions around core helium-burning stars
Hot subdwarf stars of spectral type B (sdBs) are evolved, core helium-burning
objects. The formation of those objects is puzzling, because the progenitor
star has to lose almost its entire hydrogen envelope in the red-giant phase.
Binary interactions have been invoked, but single sdBs exist as well. We report
the discovery of two close hot subdwarf binaries with small radial velocity
amplitudes. Follow-up photometry revealed reflection effects originating from
cool irradiated companions, but no eclipses. The lower mass limits for the
companions of CPD-64481 () and PHL\,457
() are significantly below the stellar mass limit. Hence
they could be brown dwarfs unless the inclination is unfavourable. Two very
similar systems have already been reported. The probability that none of them
is a brown dwarf is very small, 0.02%. Hence we provide further evidence that
substellar companions with masses that low are able to eject a common envelope
and form an sdB star. Furthermore, we find that the properties of the observed
sample of hot subdwarfs in reflection effect binaries is consistent with a
scenario where single sdBs can still be formed via common envelope events, but
their low-mass substellar companions do not survive.Comment: accepted to A&
Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor
HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass
too low to ignite core helium burning and is therefore considered as a
pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M 0.3 Msun) are
He-core objects produced by the evolution of compact binary systems. We present
in this paper a detailed abundance analysis of HD 188112 based on
high-resolution Hubble Space Telescope (HST) near and far-ultraviolet
spectroscopy. We also constrain the mass of the star's companion. We use hybrid
non-LTE model atmospheres to fit the observed spectral lines and derive the
abundances of more than a dozen elements as well as the rotational broadening
of metallic lines. We confirm the previous binary system parameters by
combining radial velocities measured in our UV spectra with the already
published ones. The system has a period of 0.60658584 days and a WD companion
with M 0.70 Msun. By assuming a tidally locked rotation, combined with
the projected rotational velocity (v sin i = 7.9 0.3 km s) we
constrain the companion mass to be between 0.9 and 1.3 Msun. We further discuss
the future evolution of the system as a potential progenitor of a
(underluminous) type Ia supernova. We measure abundances for Mg, Al, Si, P, S,
Ca, Ti, Cr, Mn, Fe, Ni, and Zn, as well as for the trans-iron elements Ga, Sn,
and Pb. In addition, we derive upper limits for the C, N, O elements and find
HD 188112 to be strongly depleted in carbon. We find evidence of non-LTE
effects on the line strength of some ionic species such as Si II and Ni II. The
metallic abundances indicate that the star is metal-poor, with an abundance
pattern most likely produced by diffusion effects.Comment: Accepted for publication in A&
Vitamin D receptor gene polymorphisms and prognosis of breast cancer among African-American and Hispanic women.
BackgroundVitamin D plays a role in cancer development and acts through the vitamin D receptor (VDR). Although African-Americans have the lowest levels of serum vitamin D, there is a dearth of information on VDR gene polymorphisms and breast cancer among African-Americans and Hispanics. This study examines whether VDR gene polymorphisms are associated with breast cancer in these cohorts.MethodsBlood was collected from 232 breast cancer patients (Cases) and 349 non-cancer subjects (Controls). Genotyping for four polymorphic variants of VDR (FokI, BsmI, TaqI and ApaI) was performed using the PCR-RFLP method.ResultsAn increased association of the VDR-Fok1 f allele with breast cancer was observed in African-Americans (OR = 1.9, p = 0.07). Furthermore, the FbTA, FbtA and fbtA haplotypes were associated with breast cancer among African-Americans (p<0.05). Latinas were more likely to have the VDR-ApaI alleles (Aa or aa) (p = 0.008). The VDR-ApaI aa genotype was significantly associated with poorly-differentiated breast tumors (p = 0.04) in combined Cases. Kaplan-Meier survival analysis showed decreased 5-year disease-free-survival (DFS) in breast cancer patients who had the VDR-Fok1 FF genotype (p<0.05). The Cox regression with multivariate analysis revealed the independent predictor value of the VDR-FokI polymorphism for DFS. The other three variants of VDR (BsmI, TaqI and ApaI) were not associated with disease outcome.ConclusionsVDR haplotypes are associated with breast cancer in African-Americans, but not in Hispanic/Latinas. The VDR-FokI FF genotype is linked with poor prognosis in African-American women with breast cancer
Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338-1925
We obtain time-series spectrophotometry observations at the VLT with the aim
of partially identifying the dominant oscillation modes in the rapidly
pulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitude
and phase variations. From the data gathered, we detect four previously known
pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2
and 2.3 % of the star's mean brightness. We also determine the atmospheric
parameters of EC 20338-1925 by fitting our non-LTE model atmospheres to an
averaged combined spectrum. The inferred parameters are Teff = 34,153+-94 K,
log g =5.966+-0.017 and log[N(He)/N(H)] = - 1.642+-0.022, where the uncertainty
estimates quoted refer to the formal fitting errors. Finally, we calculate the
observed monochromatic amplitudes and phases for the periodicities extracted
using least-squares fitting to the light curves obtained for each wavelength
bin. These observed quantities are then compared to the corresponding
theoretical values computed on the basis of dedicated model atmosphere codes
and also taking into account non-adiabatic effects. We find that the quality of
the data is sufficient to identify the dominant pulsation at 146.9 s as a
radial mode, while two of the lower amplitude periodicities must be low-degree
modes with l=0-2. This is the first time that monochromatic amplitudes and
phases have been used for mode identification in a subdwarf B star, and the
results are highly encouraging.Comment: 11 pages. Accepted for publication in Astronomy & Astrophysic
Proxima Centauri b: A Strong Case for Including Cosmic-Ray-induced Chemistry in Atmospheric Biosignature Studies
Due to its Earth-like minimum mass of 1.27 M-E and its close proximity to our solar system, Proxima Centauri b is one of the most interesting exoplanets for habitability studies. Its host star, Proxima Centauri, is however a strongly flaring star, which is expected to provide a very hostile environment for potentially habitable planets. We perform a habitability study of Proxima Centauri b assuming an Earth-like atmosphere under high stellar particle bombardment, with a focus on spectral transmission features. We employ our extensive model suite calculating energy spectra of stellar particles, their journey through the planetary magnetosphere, ionosphere, and atmosphere, ultimately providing planetary climate and spectral characteristics, as outlined in Herbst et al. Our results suggest that together with the incident stellar energy flux, high particle influxes can lead to efficient heating of the planet well into temperate climates, by limiting CH4 amounts, which would otherwise run into antigreenhouse for such planets around M stars. We identify some key spectral features relevant for future spectral observations: First, NO2 becomes the major absorber in the visible, which greatly impacts the Rayleigh slope. Second, H2O features can be masked by CH4 (near-infrared) and CO2 (mid- to far-infrared), making them nondetectable in transmission. Third, O-3 is destroyed and instead HNO3 features become clearly visible in the mid- to far-infrared. Lastly, assuming a few percent of CO2 in the atmosphere, CO2 absorption at 5.3 mu m becomes significant (for flare and nonflare cases), strongly overlapping with a flare related NO feature in Earth\u27s atmosphere
Determining the Elemental and Isotopic Composition of the preSolar Nebula from Genesis Data Analysis: The Case of Oxygen
We compare element and isotopic fractionations measured in solar wind samples
collected by NASA's Genesis mission with those predicted from models
incorporating both the ponderomotive force in the chromosphere and conservation
of the first adiabatic invariant in the low corona. Generally good agreement is
found, suggesting that these factors are consistent with the process of solar
wind fractionation. Based on bulk wind measurements, we also consider in more
detail the isotopic and elemental abundances of O. We find mild support for an
O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A
stronger conclusion must await solar wind regime specific measurements from the
Genesis samples.Comment: 6 pages, accepted by Astrophysical Journal Letter
Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456
The principal aim of this project is to determine the structural parameters
of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our
work forms part of an ongoing programme to constrain the internal
characteristics of hot B subdwarfs with the long-term goal of differentiating
between the various formation scenarios proposed for these objects. First
comparisons of asteroseismic values with evolutionary theory look promising,
however it is clear that more targets are needed for meaningful statistics to
be derived. The observational pulsation periods of PG 0911+456 were extracted
from rapid time-series photometry using standard Fourier analysis techniques.
Supplemented by spectroscopic estimates of the star's mean atmospheric
parameters, they were used as a basis for the "forward modelling" approach in
asteroseismology. The latter culminates in the identification of one or more
"optimal" models that can accurately reproduce the observed period spectrum.
This naturally leads to an identification of the oscillations detected in terms
of degree l and radial order k, and infers the structural parameters of the
target. From the photometry it was possible to extract 7 independent pulsation
periods in the 150-200 s range with amplitudes between 0.05 and 0.8 % of the
star's mean brightness. An asteroseismic search of parameter space identified
several models that matched the observed properties of PG 0911+456 well, one of
which was isolated as the "optimal" model on the basis of spectroscopic and
mode identification considerations. All the observed pulsations are identified
with low-order acoustic modes with degree indices l=0,1,2 and 4, and match the
computed periods with a dispersion of only ~0.26 %.Comment: accepted for publication in A&A, 14 pages, 13 figure
Observations and asteroseismological analysis of the rapid subdwarf B pulsator EC 09582-1137
We made photometric and spectroscopic observations of the rapidly pulsating
subdwarf B star EC 09582-1137 with the aim of determining the target's
fundamental structural parameters from asteroseismology. The new data comprise
~ 30 hours of fast time-series photometry obtained with SUSI2 at the NTT on La
Silla, Chile, as well as 1 hour of low-resolution spectroscopy gathered with
EMMI, also mounted on the NTT. From the photometry we detected 5 independent
harmonic oscillations in the 135-170 s period range with amplitudes up to 0.5%
of the mean brightness of the star. In addition, we extracted two periodicities
interpreted as components of a rotationally split multiplet that indicate a
rotation period of the order of 2-5 days. We also recovered the first harmonic
of the dominant pulsation, albeit at an amplitude below the imposed 4-sigma
detection threshold. The spectroscopic observations led to the following
estimates of the atmospheric parameters of EC 09582-1137: Teff = 34,806+-233 K,
log g = 5.80+-0.04, and log[N(He)/N(H)] = - 1.68+-0.06.
Using the observed oscillations as input, we searched in model parameter
space for unique solutions that present a good fit to the data. Under the
assumption that the two dominant observed periodicities correspond to radial or
dipole modes, we were able to isolate a well-constrained optimal model that
agrees with the atmospheric parameters derived from spectroscopy. The inferred
structural parameters of EC 09582-1137 are Teff = 34,806 K (from spectroscopy),
log g = 5.788+-0.004, M = 0.485+-0.011 M_solar, log(M_env/M_star) = -
4.39+-0.10, R = 0.147+-0.002 R_solar, and L = 28.6+-1.7 L_solar. We
additionally derive the absolute magnitude M_V = 4.44+-0.05 and the distance d
= 1460+-66 pc.Comment: accepted for publication in Astronomy and Astrophysic
Search for Pairs of Isolated Radio Pulsars - Components in Disrupted Binary Systems
We have developed a method for analyzing the kinematic association of
isolated relativistic objects - possible remnants of disrupted close binary
systems. We investigate pairs of fairly young radio pulsars with known proper
motions and estimated distances (dispersion measures) that are spaced no more
than 2-3 kpc apart. Using a specified radial velocity distribution for these
objects, we have constructed 100-300 thousand trajectories of their possible
motion in the Galactic gravitational field on a time scale of several million
years. The probabilities of their close encounters at epochs consistent with
the age of the younger pulsar in the pair are analyzed. When these
probabilities exceed considerably their reference values obtained by assuming a
purely random encounter between the pulsars under consideration, we conclude
that the objects may have been gravitationally bound in the past. As a result,
we have detected six pulsar pairs (J0543+2329/J0528+2200,
J1453-6413/J1430-6623, J2354+6155/J2321+6024, J1915+1009/J1909+1102,
J1832-0827/J1836-1008, and J1917+1353/J1926+1648) that are companions in
disrupted binary systems with a high probability. Estimates of their kinematic
ages and velocities at binary disruption and at the present epoch are provided
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