394 research outputs found
Variability of eta Carinae III
Spectra (1951-78) of the central object in eta Car, taken by A.D. Thackeray,
reveal three previously unrecorded epochs of low excitation. Since 1948, at
least, these states have occurred regularly in the 2020 day cycle proposed by
Damineli et al. They last about 10 percent of each cycle. Early slit spectra
(1899-1919) suggest that at that time the object was always in a low state.
JHKL photometry is reported for the period 1994-2000. This shows that the
secular increase in brightness found in 1972-94 has continued and its rate has
increased at the shorter wavelengths. Modulation of the infrared brightness in
a period near 2020 days continues. There is a dip in the JHKL light curves near
1998.0, coincident with a dip in the X-ray light curve. Evidence is given that
this dip in the infrared repeats in the 2020 day cycle. As suggested by
Whitelock & Laney, the dip is best interpreted as an eclipse phenomenon in an
interacting binary system; the object eclipsed being a bright region (`hot
spot'), possibly on a circumstellar disc or produced by interacting stellar
winds. The eclipse coincides in phase and duration with the state of low
excitation. It is presumably caused by a plasma column and/or by one of the
stars in the system.Comment: 10 pages, 7 postscript figures, accepted for MNRA
Eta Carinae in the Context of the Most Massive Stars
Eta Car, with its historical outbursts, visible ejecta and massive, variable
winds, continues to challenge both observers and modelers. In just the past
five years over 100 papers have been published on this fascinating object. We
now know it to be a massive binary system with a 5.54-year period. In January
2009, Eta Car underwent one of its periodic low-states, associated with
periastron passage of the two massive stars. This event was monitored by an
intensive multi-wavelength campaign ranging from gamma-rays to radio. A large
amount of data was collected to test a number of evolving models including 3-D
models of the massive interacting winds. August 2009 was an excellent time for
observers and theorists to come together and review the accumulated studies, as
have occurred in four meetings since 1998 devoted to Eta Car. Indeed, Eta Car
behaved both predictably and unpredictably during this most recent periastron,
spurring timely discussions. Coincidently, WR140 also passed through periastron
in early 2009. It, too, is a intensively studied massive interacting binary.
Comparison of its properties, as well as the properties of other massive stars,
with those of Eta Car is very instructive. These well-known examples of evolved
massive binary systems provide many clues as to the fate of the most massive
stars. What are the effects of the interacting winds, of individual stellar
rotation, and of the circumstellar material on what we see as
hypernovae/supernovae? We hope to learn.Comment: 26 pages, 7 figures, IAU General Assembly Joint Discussion on Eta
Carinae in Context of the Most Massive Star
Origens da vida
We present a historical perspective about the ideas concerning the origin of life. After displaying the main topics necessary for understanding lifes origin, the main characteristics of the present life forms and their relationships are shown, suggesting a common ancestor. The conditions for prebiotic chemistry in terrestrial or interplanetary environments are reviewed. We put in context the arguments about the early origin of replicators versus metabolism. The very narrow window for life settlement in the early Earth is stressed, pointing to the likelihood of life arising in other places in the Universe. Finally, we present the cornerstones of current search for life outside our planet.Apresentamos um panorama geral das idĂ©ias sobre a origem da vida na histĂłria. ApĂłs delinearmos os principais temas necessários para o entendimento da origem da vida, mostramos as caracterĂsticas básicas da vida atual e as relações entre os seres vivos que sugerem um ancestral comum. Revemos as condições quĂmicas prĂ©-biĂłticas no ambiente terrestre e fora dele. Quanto ao aparecimento da vida, confrontamos as principais vertentes sobre a precedĂŞncia do cĂłdigo genĂ©tico ou do metabolismo. Evidenciamos a curta janela de tempo para o estabelecimento da vida na Terra, indicando a facilidade de esse processo ocorrer no Universo. Finalmente, sĂŁo apresentados os fundamentos dos projetos atuais de procura de vida fora de nosso planeta
Prediction for the He I 10830A Absorption Wing in the Coming Event of Eta Carinae
We propose an explanation to the puzzling appearance of a wide blue
absorption wing in the He I 10830A P-Cygni profile of the massive binary star
Eta Carinae several months before periastron passage. Our basic assumption is
that the colliding winds region is responsible for the blue wing absorption. By
fitting observations, we find that the maximum outflow velocity of this
absorbing material is ~2300 km/s. We also assume that the secondary star is
toward the observer at periastron passage. With a toy-model we achieve two
significant results. (1) We show that the semimajor axis orientation we use can
account for the appearance and evolution of the wide blue wing under our basic
assumption. (2) We predict that the Doppler shift (the edge of the absorption
profile) will reach a maximum 0-3 weeks before periastron passage, and not
necessarily exactly at periastron passage or after periastron passage.Comment: 15 pages, 6 figures. Accepted for publication in MNRA
Long-Term Optical Monitoring of Eta Carinae. Multiband light curves for a complete orbital period
The periodicity of 5.5 years for some observational events occurring in Eta
Carinae manifests itself across a large wavelength range and has been
associated with its binary nature. These events are supposed to occur when the
binary components are close to periastron. To detect the previous periastron
passage of Eta Car in 2003, we started an intensive, ground-based, optical,
photometric observing campaign. We continued observing the object to monitor
its photometric behavior and variability across the entire orbital cycle. Our
observation program consisted of daily differential photometry from CCD images,
which were acquired using a 0.8 m telescope and a standard BVRI filter set at
La Plata Observatory. The photometry includes the central object and the
surrounding Homunculus nebula. We present up-to-date results of our observing
program, including homogeneous photometric data collected between 2003 and
2008. Our observations demonstrated that Eta Car has continued increasing in
brightness at a constant rate since 1998. In 2006, it reached its brightest
magnitude (V ~ 4.7) since about 1860s. The object then suddenly reverted its
brightening trend, fading to V = 5.0 at the beginning of 2007, and has
maintained a quite steady state since then. We continue the photometric
monitoring of Eta Car in anticipation of the next "periastron passage",
predicted to occur at the beginning of 2009.Comment: Accepted by A&A. The paper contains 3 figures and 2 table
Time variations of the narrow FeII and HI spectral emission lines from the close vicinity of Eta Carinae during the spectral event of 2003
The spectrum of Eta Carinae and its ejecta shows slow variations over a
period of 5.5 years. However, the spectrum changes drastically on a time scale
of days once every period called the 'spectral event'. We report on variations
in the narrow emission line spectrum of gas condensations (the Weigelt blobs)
close to the central star during a spectral event. The rapid changes in the
stellar radiation field illuminating the blobs make the blobs a natural
astrophysical laboratory to study atomic photoprocesses. The different
responses of the HI Paschen lines, fluorescent lines and forbidden
[FeII] lines allow us to identify the processes and estimate physical
conditions in the blobs. This paper is based on observations from the Pico dos
Dias Observatory (LNA/Brazil) during the previous event in June 2003.Comment: Accepted for publication in A&
The Stellar Content of Obscured Galactic Giant H II Regions: II. W42
We present near infrared J, H, and K images and K-band spectroscopy in the
giant HII region W42. A massive star cluster is revealed; the color-color plot
and K-band spectroscopic morphology of two of the brighter objects suggest the
presence of young stellar objects. The spectrum of the bright central star is
similar to unobscured stars with MK spectral types of O5-O6.5. If this star is
on the zero age main sequence, then the derived spectrophotometric distance is
considerably smaller than previous estimates. The Lyman continuum luminosity of
the cluster is a few times that of the Trapezium. The slope of the K-band
luminosity function is similar to that for the Trapezium cluster and
significantly steeper than that for the massive star cluster in M17 or the
Arches cluster near the Galactic center.Comment: 30 pages, 11 figures, late
A Model for the Strings of Eta Carinae
We propose a model based on ionization shadows to explain the formation of
the long and narrow strings of Eta Carinae. Five strings are known, all located
along the symmetry axis outside the Homunculus. The model assumes that each
string is formed in a shadow behind a dense clump near the symmetry axis. The
surrounding gas is ionized first, becomes much hotter, and compresses the gas
in the shadow. This leads to the formation of a radial, dense, long, and narrow
region, i.e., a string. Later the neutral material in the strings is ionized,
and becomes brighter. Still later it re-expands, and we predict that in about
200 years the strings will fade. The condition for the model to work is that
the ionization front, due to the diffuse ionizing recombination radiation of
the surrounding gas, proceeds into the shadow at a velocity slower than the
compression speed. From that we get a condition on the mass loss rate of the
mass loss episode that formed the strings, which should be less than 10^{-4}
Mo/year. The model can also explain the strings in the planetary nebula NGC
6543.Comment: 8 pages; Submitted to A&
Particle acceleration in the expanding blast wave of Eta Carina's Great Eruption of 1843
Non-thermal hard X-ray and high-energy (HE; 1 MeV < E < 100 GeV) gamma-ray
emission in the direction of Eta Carina has been recently detected using the
INTEGRAL, AGILE and Fermi satellites. So far this emission has been interpreted
in the framework of particle acceleration in the colliding wind region between
the two massive stars. However, the existence of a very fast moving blast wave
which originates in the historical 1843 "Great Eruption" provides an
alternative particle acceleration site in this system. Here we explore an
alternate scenario and find that inverse Compton emission from electrons
accelerated in the blast wave can naturally explain both the flux and spectral
shape of the measured hard X-ray and HE gamma-ray emission. This scenario is
further supported by the lack of significant variability in the INTEGRAL and
Fermi measured fluxes.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letters; v2:
corrected SED and discussion in Section 4 accordingl
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