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Register a new userDiscovery of CH and OH in the -513 km s-1 Ejecta of Eta Carinae
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The very massive star, Eta Carinae, is enshrouded in an unusual complex of stellar ejecta, which is highly depleted in C and O, and enriched in He and N. This circumstellar gas gives rise to distinct absorption components corresponding to at least 20 different velocities along the line-of-sight. The velocity component at -513 kms-1 exhibits very low ionization with predominantly neutral species of iron-peak elements. Our statistical equilibrium/photoionization modeling indicates that the low temperature (T = 760 K) and high density (n_H=10^7 cm^-3) of the -513 kms-1 component is conducive to molecule formation including those with the elements C and O. Examination of echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the confirms the models predictions. The molecules, H_2, CH, and most likely OH, have been identified in the -513 kms-1 absorption spectrum. This paper presents the analysis of the HST/STIS spectra with the deduced column densities for CH, OH and C I, and upper limit for CO. It is quite extraordinary to see molecular species in a cool environment at such a high velocity. The sharp molecular and ionic absorptions in this extensively CNO- processed material offers us a unique environment for studying the chemistry, dust formation processes, and nucleosynthesis in the ejected layers of a highly evolved massive star.
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The nebula around eta Carinae consists of two distinct parts: the Homunculus and the outer ejecta. The outer ejecta are mainly a collection of numerous filaments, shaped irregularly and distributed over an area of 1arcminx1arcmin. While the Homunculus is mainly a reflection nebula, the outer ejecta are an emission nebula. Kinematic analysis of the outer ejecta (as the Homunculus) show their bi-directional expansion. Radial velocities in the outer ejecta reach up to >2000km/s and the gas gives rise to X-ray emission. The temperature of the X-ray gas is of the order of 0.65 keV. These shock temperatures indicate velocities of the shocking gas of 750km/s, about what was found for the average expansion velocity of the outer ejecta. HST/STIS data from the strings, long, highly collimated structures in the outer ejecta, show that the electron density of the strings is of the order of 10^4cm^-3 Other structures in the outer ejecta show similar values. String 1 has a mass of about 3 10^-4M_sun, a density gradient along the strings or a denser leading head was not found.
The outer ejecta is part of the nebula around Eta Carinae. They are filamentary, shaped irregularly and larger than the Homunculus, the central bipolar nebula. While the Homuculus is mainly a reflection nebula, the outer ejecta is an emission structure. However, we showed with kinematic analysis that the outer ejecta (as the Homunculus) expands bi-directional despite of its complex morphology. Radial velocities in the outer ejecta reach up to 2000km/s and give rise to X-ray emission. An analysis showing the distribution of the soft X-ray emission and its comparison to the optical emitting gas is presented here. X-ray maxima are found in areas in which the expansion velocities are highest. The temperature of 0.65 keV determined with the CHANDRA/ACIS data and thermal equilibrium models indicates post-shock velocities of 750km/s, about what was found in the spectra. In addition analysis of the new HST-STIS data from the Strings--long, highly collimated structures in the outer ejecta--are presented. The data show that the electron density of the Strings is of the order of 10^4 cm^-3. The same value was detected for other structures in the outer ejecta. With this density String 1 has a mass of about 3 10^-4 M_sun and the total ejecta could be as massive as 0.5 M_sun.
Previous submillimetre (submm) observations detected 0.7 solar masses of cool dust emission around the Luminous Blue Variable (LBV) star Eta Carinae. These observations were hindered by the low declination of Eta Carinae and contamination from free-free emission orginating from the stellar wind. Here, we present deep submm observations with LABOCA at 870um, taken shortly after a maximum in the 5.5-yr radio cycle. We find a significant difference in the submm flux measured here compared with the previous measurement: the first indication of variability at submm wavelengths. A comparison of the submm structures with ionised emission features suggests the 870um is dominated by emission from the ionised wind and not thermal emission from dust. We estimate 0.4 +/- 0.1 solar masses of dust surrounding Eta Carinae. The spatial distribution of the submm emission limits the mass loss to within the last thousand years, and is associated with mass ejected during the great eruptions and the pre-outburst LBV wind phase; we estimate that Eta Carinae has ejected > 40 solar masses of gas within this timescale.
Using the high dispersion NUV mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) to observe Eta Carinae, we have resolved and identified over 500 sharp, circumstellar absorption lines of iron-group singly-ionized and neutral elements with ~20 velocity components ranging from -146 km/s to -585 km/s. These lines are from transitions originating from ground and metastable levels as high as 40,000 cm-1 above ground. The absorbing material is located either in dense inhomogeneities in the stellar wind, the warm circumstellar gas immediately in the vicinity of Eta Carinae, or within the cooler foreground lobe of the Homunculus. We have used classical curve-of-growth analysis to derive atomic level populations for FeII at -146 km/s and for TiII at -513 km/s. These populations, plus photoionization and statistical equilibrium modeling, provide electron temperatures, Te, densities, n, and constraints on distances from the stellar source, r. For the -146 km/s component, we derive Te = 6400 K, n(H)>10e7 - 10e8 cm-3, and d ~1300 AU. For the -513 km/s component, we find a much cooler temperature, Te= 760 K, with n(H)> 10e7cm-3, we estimate d~10,000 AU. The large distances for these two components place the absorptions in the vicinity of identifiable ejecta from historical events, not near or in the dense wind of Eta Carinae. Further analysis, in parallel with obtaining improved experimental and theoretical atomic data, is underway to determine what physical mechanisms and elemental abundances can explain the large number of strong circumstellar absorption features in the spectrum of Eta Carinae.
The late-stage evolution of very massive stars such as $eta$ Carinae may be dominated by episodic mass ejections which may later lead to Type II superluminous supernova (SLSN-II; e.g., SN 2006gy). However, as long as $eta$ Car is one of a kind, it is nearly impossible to quantitatively evaluate these possibilities. Here we announce the discovery of five objects in the nearby ($sim4-8$ Mpc) massive star-forming galaxies M51, M83, M101 and NGC6946 that have optical through mid-IR photometric properties consistent with the hitherto unique $eta$ Car. The Spitzer mid-IR spectral energy distributions of these $L_{bol}simeq3-8times10^{6} L_odot$ objects rise steeply in the $3.6-8 mu$m bands, then turn over between $8$ and $24 mu$m, indicating the presence of warm ($sim400-600$ K) circumstellar dust. Their optical counterparts in HST images are $sim1.5-2$ dex fainter than their mid-IR peaks and require the presence of $sim5-10 M_odot$ of obscuring material. Our finding implies that the rate of $eta$ Car-like events is a fraction $f=0.094$ ($0.040 < f < 0.21$ at $90%$ confidence) of the core-collapse supernova (ccSN) rate. If there is only one eruption mechanism and SLSN-II are due to ccSN occurring inside these dense shells, then the ejection mechanism is likely associated with the onset of carbon burning ($sim 10^3 - 10^4$ years) which is also consistent with the apparent ages of massive Galactic shells.
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