317 research outputs found
The Nearby Neutron Star RX J0720.4-3125 from Radio to X-rays
We present radio, optical, ultraviolet, and X-ray observations of the
isolated, thermally-emitting neutron star RX J0720.4-3125 using the Parkes
radio telescope, the Very Large Array, the Hubble Space Telescope, and the
Chandra X-ray Observatory. From these data we show that the optical/UV spectrum
of RX J0720.4-3125 is not well fit by a Rayleigh-Jeans tail as previously
thought, but is instead best fit by either a single non-thermal power-law or a
combination of a Rayleigh-Jeans tail and a non-thermal power-law. Taken
together with the X-ray spectrum, we find the best model for RX J0720.4-3125 to
be two blackbodies plus a power-law, with the cool blackbody implying a radius
of 11-13 km at an assumed distance of 300 pc. This is similar to many middle
aged (10^{5-6} yr) radio pulsars such as PSR B0656+14, evidence supporting the
hypothesis that RX J0720.4-3125 is likely to be an off-beam radio pulsar. The
radio data limit the flux at 1.4 GHz to be <0.24 mJy, or a luminosity limit of
4*pi*d^2*F < 3e25*d_300^2 ergs/s, and we see no sign of extended nebulosity,
consistent with expectations for a pulsar like RX J0720.4-3125.Comment: 13 pages, 9 figures. Uses emulateapj5.sty and onecolfloat5.sty.
Accepted for publication in Ap
The XO Project: Searching for Transiting Extra-solar Planet Candidates
The XO project's first objective is to find hot Jupiters transiting bright
stars, i.e. V < 12, by precision differential photometry. Two XO cameras have
been operating since September 2003 on the 10,000-foot Haleakala summit on
Maui. Each XO camera consists of a 200-mm f/1.8 lens coupled to a 1024x1024
pixel, thinned CCD operated by drift scanning. In its first year of routine
operation, XO has observed 6.6% of the sky, within six 7 deg-wide strips
scanned from 0 deg to +63 deg of declination and centered at RA=0, 4, 8, 12,
16, and 20 hours. Autonomously operating, XO records 1 billion pixels per clear
night, calibrates them photometrically and astrometrically, performs aperture
photometry, archives the pixel data and transmits the photometric data to STScI
for further analysis. From the first year of operation, the resulting database
consists of photometry of 100,000 stars at more than 1000 epochs per star with
differential photometric precision better than 1% per epoch. Analysis of the
light curves of those stars produces transiting-planet candidates requiring
detailed follow up, described elsewhere, culminating in spectroscopy to measure
radial-velocity variation in order to differentiate genuine planets from the
more numerous impostors, primarily eclipsing binary and multiple stars.Comment: 29 pages, 12 figures, accepted by PASP for Aug 2005 issu
A Transiting Planet of a Sun-like Star
A planet transits an 11th magnitude, G1V star in the constellation Corona
Borealis. We designate the planet XO-1b, and the star, XO-1, also known as GSC
02041-01657. XO-1 lacks a trigonometric distance; we estimate it to be 200+-20
pc. Of the ten stars currently known to host extrasolar transiting planets, the
star XO-1 is the most similar to the Sun in its physical characteristics: its
radius is 1.0+-0.08 R_Sun, its mass is 1.0+-0.03 M_Sun, V sini < 3 km/s, and
its metallicity [Fe/H] is 0.015+-0.04. The orbital period of the planet XO-1b
is 3.941534+-0.000027 days, one of the longer ones known. The planetary mass is
0.90+-0.07 M_Jupiter, which is marginally larger than that of other transiting
planets with periods between 3 and 4 days. Both the planetary radius and the
inclination are functions of the spectroscopically determined stellar radius.
If the stellar radius is 1.0+-0.08 R_Sun, then the planetary radius is
1.30+-0.11 R_Jupiter and the inclination of the orbit is 87.7+-1.2 degrees. We
have demonstrated a productive international collaboration between professional
and amateur astronomers that was important to distinguishing this planet from
many other similar candidates.Comment: 31 pages, 9 figures, accepted for part 1 of Ap
Hot subdwarfs from the ESO Supernova Ia Progenitor Survey: II. Atmospheric parameters of subdwarf O stars
We address the origin and evolutionary status of hot subdwarf stars by
studying the optical spectral properties of 58 subdwarf O (sdO) stars.
Combining them with the results of our previously studied subdwarf B (sdB)
stars, we aim at investigating possible evolutionary links. We analyze
high-resolution ESO VLT UVES spectra from the ESO Supernova Ia Progenitor
Survey (SPY). Effective temperatures, gravities, and helium abundances are
determined simultaneously by fitting the profiles of H and He lines using
dedicated synthetic spectra in NLTE. Evidence for cool companions to 8 sdOs as
well as a binary consisting of two sdO stars is found. A correlation between He
abundances and the presence of carbon and/or nitrogen lines emerges: below
solar He abundance, no sdO shows C or N lines. In contrast, C and/or N lines
are present in ALL sdOs with super- solar He abundance. We thus use the solar
He abundance to divide our sample into He-deficient and He-enriched sdOs. While
He-deficient sdOs are scattered in a wide range of the Teff-log(g)-diagram,
most of the He-enriched sdOs cluster in a narrow region at Teff = 40,000 ...
50,000K and log(g)=5.5 ... 6.0. An evolu- tionary link between sdBs and sdOs
appears plausible only for the He-deficient sdOs indicating that they are the
likely successors to sdBs. The properties of He-enriched sdOs cannot be
explained with canonical single star evolutionary models. Alternative scenarios
(late hot flasher) as well as for binary evolution (white dwarf merger;
post-RGB evolution) are tested. While we regard the post-RGB scenario as
inappropriate, the white dwarf merger and the late hot flasher scenarios remain
viable to explain the origin of He-enriched sdOs.Comment: 14 pages, 10 figures, Astronomy & Astrophysics accepte
Loss of lag-response curvilinearity of indices of heart rate variability in congestive heart failure
BACKGROUND: Heart rate variability (HRV) is known to be impaired in patients with congestive heart failure (CHF). Time-domain analysis of ECG signals traditionally relies heavily on linear indices of an essentially non-linear phenomenon. Poincaré plots are commonly used to study non-linear behavior of physiologic signals. Lagged Poincaré plots incorporate autocovariance information and analysis of Poincaré plots for various lags can provide interesting insights into the autonomic control of the heart. METHODS: Using Poincaré plot analysis, we assessed whether the relation of the lag between heart beats and HRV is altered in CHF. We studied the influence of lag on estimates of Poincaré plot indices for various lengths of beat sequence in a public domain data set (PhysioNet) of 29 subjects with CHF and 54 subjects with normal sinus rhythm. RESULTS: A curvilinear association was observed between lag and Poincaré plot indices (SD1, SD2, SDLD and SD1/SD2 ratio) in normal subjects even for a small sequence of 50 beats (p value for quadratic term 3 à 10(-5), 0.002, 3.5 à 10(-5 )and 0.0003, respectively). This curvilinearity was lost in patients with CHF even after exploring sequences up to 50,000 beats (p values for quadratic term > 0.5). CONCLUSION: Since lagged Poincaré plots incorporate autocovariance information, these analyses provide insights into the autonomic control of heart rate that is influenced by the non-linearity of the signal. The differences in lag-response in CHF patients and normal subjects exist even in the face of the treatment received by the CHF patients
Mitochondrial changes within axons in multiple sclerosis
Multiple sclerosis is the most common cause of non-traumatic neurological impairment in young adults. An energy deficient state has been implicated in the degeneration of axons, the pathological correlate of disease progression, in multiple sclerosis. Mitochondria are the most efficient producers of energy and play an important role in calcium homeostasis. We analysed the density and function of mitochondria using immunohistochemistry and histochemistry, respectively, in chronic active and inactive lesions in progressive multiple sclerosis. As shown before in acute pattern III and Baloâs lesions, the mitochondrial respiratory chain complex IV activity is reduced despite the presence of mitochondria in demyelinated axons with amyloid precursor protein accumulation, which are predominantly located at the active edge of chronic active lesions. Furthermore, the strong non-phosphorylated neurofilament (SMI32) reactivity was associated with a significant reduction in complex IV activity and mitochondria within demyelinated axons. The complex IV defect associated with axonal injury may be mediated by soluble products of innate immunity, as suggested by an inverse correlation between complex IV activity and macrophage/microglial density in chronic lesions. However, in inactive areas of chronic multiple sclerosis lesions the mitochondrial respiratory chain complex IV activity and mitochondrial mass, judged by porin immunoreactivity, are increased within approximately half of large (>2.5 ÎŒm diameter) chronically demyelinated axons compared with large myelinated axons in the brain and spinal cord. The axon-specific mitochondrial docking protein (syntaphilin) and phosphorylated neurofilament-H were increased in chronic lesions. The lack of complex IV activity in a proportion of Na+/K+ ATPase α-1 positive demyelinated axons supports axonal dysfunction as a contributor to neurological impairment and disease progression. Furthermore, in vitro studies show that inhibition of complex IV augments glutamate-mediated axonal injury (amyloid precursor protein and SMI32 reactivity). Our findings have important implications for both axonal degeneration and dysfunction during the progressive stage of multiple sclerosis
One year follow-up of patients with refractory angina pectoris treated with enhanced external counterpulsation
BACKGROUND: Enhanced external counterpulsation (EECP) is a non-invasive technique that has been shown to be effective in reducing both angina and myocardial ischemia in patients not responding to medical therapy and without revascularization alternatives. The aim of the present study was to assess the long-term outcome of EECP treatment at a Scandinavian centre, in relieving angina in patients with chronic refractory angina pectoris. METHODS: 55 patients were treated with EECP. Canadian cardiovascular society (CCS) class, antianginal medication and adverse clinical events were collected prior to EECP, at the end of the treatment, and at six and 12 months after EECP treatment. Clinical signs and symptoms were recorded. RESULTS: EECP treatment significantly improved the CCS class in 79 ± 6% of the patients with chronic angina pectoris (p < 0.001). The reduction in CCS angina class was seen in patients with CCS class III and IV and persisted 12 months after EECP treatment. There was no significant relief in angina in patients with CCS class II prior to EECP treatment. 73 ± 7% of the patients with a reduction in CCS class after EECP treatment improved one CCS class, and 22 ± 7% of the patients improved two CCS classes. The improvement of two CCS classes could progress over a six months period and tended to be more prominent in patients with CCS class IV. In accordance with the reduction in CCS classes there was a significant decrease in the weekly nitroglycerin usage (p < 0.05). CONCLUSION: The results from the present study show that EECP is a safe treatment for highly symptomatic patients with refractory angina. The beneficial effects were sustained during a 12-months follow-up period
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