970 research outputs found
Photometric Observations of the Eta Carinae 2009.0 Spectroscopic Event
We have observed Eta Carinae over 34 nights between 4th January 2009 and 27th
March 2009 covering the estimated timeframe for a predicted spectroscopic event
related to a suspected binary system concealed within the homunculus nebula. A
photometric minimum feature was confirmed to be periodic and comparison to a
previous event indicated that the period to within our error at 2022.6 +/-1.0
d. Using the E-region standard star system, the apparent V magnitudes
determined for the local comparison stars were HD303308 8.14+/-0.02, HD 93205
7.77 +/-0.03 and HD93162 8.22 +/-0.05. The latter star was found to be dimmer
than previously reported.Comment: 5 pages,4 figures, 1 tabl
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event: III. the He II 4686 Line
We continue to explore the accretion model of the massive binary system eta
Carinae by studying the anomalously high He II 4686 line. The line appears just
before periastron and disappears immediately thereafter. Based on the He II
4686 line emission from O-stars and their modeling in the literature, we
postulate that the He II 4686 line comes from the acceleration zone of the
secondary stellar wind. We attribute the large increase in the line intensity
to a slight increase in the density of the secondary stellar wind in its
acceleration zone. The increase in density could be due to the ionization and
subsequent deceleration of the wind by the enhanced X-ray emission arising from
the shocked secondary wind further downstream or to accretion of the primary
stellar wind. Accretion around the secondary equatorial plane gives rise to
collimation of the secondary wind, which increases its density, hence enhancing
the He II 4686 emission line. In contrast with previous explanations, the
presently proposed model does not require a prohibitively high X-ray flux to
directly photoionize the He.Comment: ApJ, in pres
Accretion onto the Companion of Eta Carinae During the Spectroscopic Event. IV. the Disappearance of Highly Ionized Lines
We show that the rapid and large decrease in the intensity of high-ionization
emission lines from the Eta Carinae massive binary system can be explained by
the accretion model. These emission lines are emitted by material in the nebula
around the binary system that is being ionized by radiation from the hot
secondary star. The emission lines suffer three months long deep fading every
5.54 year, assumed to be the orbital period of the binary system. In the
accretion model, for ~70 day the less massive secondary star is accreting mass
from the primary wind instead of blowing its fast wind. The accretion event has
two effects that substantially reduce the high-energy ionizing radiation flux
from the secondary star. (1) The accreted mass absorbs a larger fraction of the
ionizing flux. (2) The accreted mass forms a temporarily blanked around the
secondary star that increases its effective radius, hence lowering its
effective temperature and the flux of high energy photons. This explanation is
compatible with the fading of the emission lines at the same time the X-ray is
declining to its minimum, and with the fading being less pronounced in the
polar directions.Comment: ApJ, in pres
On the photometric variability of blue supergiants in NGC 300 and its impact on the Flux-weighted Gravity-Luminosity Relationship
We present a study of the photometric variability of spectroscopically
confirmed supergiants in NGC 300, comprising 28 epochs extending over a period
of five months. We find 15 clearly photometrically variable blue supergiants in
a sample of nearly 70 such stars, showing maximum light amplitudes ranging from
0.08 to 0.23 magnitudes in the V band, and one variable red supergiant. We show
their light curves, and determine semi-periods for two A2 Ia stars. Assuming
that the observed changes correspond to similar variations in the bolometric
luminosity, we test for the influence of this variability on the Flux-weighted
Gravity--Luminosity Relationship and find a negligible effect, showing that the
calibration of this relationship, which has the potential to measure
extragalactic distances at the Cepheid accuracy level, is not affected by the
stellar photometric variability in any significant way.Comment: 9 pages, 3 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
The Purple Haze of Eta Carinae: Binary-Induced Variability?
Asymmetric variability in ultraviolet images of the Homunculus obtained with
the Advanced Camera for Surveys/High Resolution Camera on the Hubble Space
Telescope suggests that Eta Carinae is indeed a binary system. Images obtained
before, during, and after the recent ``spectroscopic event'' in 2003.5 show
alternating patterns of bright spots and shadows on opposite sides of the star
before and after the event, providing a strong geometric argument for an
azimuthally-evolving, asymmetric UV radiation field as one might predict in
some binary models. The simplest interpretation of these UV images, where
excess UV escapes from the secondary star in the direction away from the
primary, places the major axis of the eccentric orbit roughly perpendicular to
our line of sight, sharing the same equatorial plane as the Homunculus, and
with apastron for the hot secondary star oriented toward the southwest of the
primary. However, other orbital orientations may be allowed with more
complicated geometries. Selective UV illumination of the wind and ejecta may be
partly responsible for line profile variations seen in spectra. The brightness
asymmetries cannot be explained plausibly with delays due to light travel time
alone, so a single-star model would require a seriously asymmetric shell
ejection.Comment: 8 pages, fig 1 in color, accepted by ApJ Letter
Using Artificial Intelligence to Predict Intracranial Hypertension in Patients After Traumatic Brain Injury:A Systematic Review
Intracranial hypertension (IH) is a key driver of secondary brain injury in patients with traumatic brain injury. Lowering intracranial pressure (ICP) as soon as IH occurs is important, but a preemptive approach would be more beneficial. We systematically reviewed the artificial intelligence (AI) models, variables, performances, risks of bias, and clinical machine learning (ML) readiness levels of IH prediction models using AI. We conducted a systematic search until 12-03-2023 in three databases. Only studies predicting IH or ICP in patients with traumatic brain injury with a validation of the AI model were included. We extracted type of AI model, prediction variables, model performance, validation type, and prediction window length. Risk of bias was assessed with the Prediction Model Risk of Bias Assessment Tool, and we determined the clinical ML readiness level. Eleven out of 399 nonduplicate publications were included. A gaussian processes model using ICP and mean arterial pressure was most common. The maximum reported area under the receiver operating characteristic curve was 0.94. Four studies conducted external validation, and one study a prospective clinical validation. The prediction window length preceding IH varied between 30 and 60 min. Most studies (73%) had high risk of bias. The highest clinical ML readiness level was 6 of 9, indicating âreal-time model testingâ stage in one study. Several IH prediction models using AI performed well, were externally validated, and appeared ready to be tested in the clinical workflow (clinical ML readiness level 5 of 9). A Gaussian processes model was most used, and ICP and mean arterial pressure were frequently used variables. However, most studies showed a high risk of bias. Our findings may help position AI for IH prediction on the path to ultimate clinical integration and thereby guide researchers plan and design future studies.</p
Using Artificial Intelligence to Predict Intracranial Hypertension in Patients After Traumatic Brain Injury:A Systematic Review
Intracranial hypertension (IH) is a key driver of secondary brain injury in patients with traumatic brain injury. Lowering intracranial pressure (ICP) as soon as IH occurs is important, but a preemptive approach would be more beneficial. We systematically reviewed the artificial intelligence (AI) models, variables, performances, risks of bias, and clinical machine learning (ML) readiness levels of IH prediction models using AI. We conducted a systematic search until 12-03-2023 in three databases. Only studies predicting IH or ICP in patients with traumatic brain injury with a validation of the AI model were included. We extracted type of AI model, prediction variables, model performance, validation type, and prediction window length. Risk of bias was assessed with the Prediction Model Risk of Bias Assessment Tool, and we determined the clinical ML readiness level. Eleven out of 399 nonduplicate publications were included. A gaussian processes model using ICP and mean arterial pressure was most common. The maximum reported area under the receiver operating characteristic curve was 0.94. Four studies conducted external validation, and one study a prospective clinical validation. The prediction window length preceding IH varied between 30 and 60 min. Most studies (73%) had high risk of bias. The highest clinical ML readiness level was 6 of 9, indicating âreal-time model testingâ stage in one study. Several IH prediction models using AI performed well, were externally validated, and appeared ready to be tested in the clinical workflow (clinical ML readiness level 5 of 9). A Gaussian processes model was most used, and ICP and mean arterial pressure were frequently used variables. However, most studies showed a high risk of bias. Our findings may help position AI for IH prediction on the path to ultimate clinical integration and thereby guide researchers plan and design future studies.</p
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