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Register a new userA High-Resolution Study of Eta Carinaes Outer Ejecta
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Authors
Kerstin Weis
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Eta Carinae is a very luminous and unstable evolved star. Outflowing material ejected during the stars giant eruption in 1843 surrounds it as a nebula which consists of an inner bipolar region(the Homunculus) and the Outer Ejecta. The outer ejecta is very filamentary and shaped irregularly. Kinematic analysis, however, shows a regular bi-directional expansion despite of the complex morphology. Radial velocities in the outer ejecta reach up to 2000 kms/s and give rise to X-ray emission first detected by ROSAT. We will present a detailed study of the outer ejecta based on HST images, high-resolution echelle spectra for kinematic studies, images from CHANDRA/ACIS and HST-STIS spectra.
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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.
In our ongoing study of eta Carinaes light echoes, there is a relatively bright echo that has been fading slowly, reflecting the 1845-1858 plateau of the eruption. A separate paper discusses its detailed evolution, but here we highlight one important result: the H-alpha line shows extremely broad emission wings that reach -10,000km/s to the blue and +20,000km/s to the red. The line profile shape is inconsistent with electron scattering wings, indicating high-velocity outflowing material. These are the fastest outflow speeds ever seen in a non-terminal massive star eruption. The broad wings are absent in early phases of the eruption, but strengthen in the 1850s. These speeds are two orders of magnitude faster than the escape speed from a warm supergiant, and 5-10 times faster than winds from O-type or Wolf-Rayet stars. Instead, they are reminiscent of fast supernova ejecta or outflows from accreting compact objects, profoundly impacting our understanding of eta Car and related transients. This echo views eta Car from latitudes near the equator, so the high speed does not trace a collimated polar jet aligned with the Homunculus. Combined with fast material in the Outer Ejecta, it indicates a wide-angle explosive outflow. The fast material may constitute a small fraction of the total outflowing mass, most of which expands at 600 km/s. This is reminiscent of fast material revealed by broad absorption during the presupernova eruptions of SN2009ip.
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.
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.
Aims. The structural inhomogeneities and kinematics of massive star nebulae are tracers of their mass-loss history. We conduct a three-dimensional morpho-kinematic analysis of the ejecta of eta Car outside its famous Homunculus nebula. Methods. We carried out the first large-scale integral field unit observations of eta Car in the optical, covering a field of view of 1x1 centered on the star. Observations with the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) reveal the detailed three-dimensional structure of eta Cars outer ejecta. Morpho-kinematic modeling of these ejecta is conducted with the code SHAPE. Results. The largest coherent structure in eta Cars outer ejecta can be described as a bent cylinder with roughly the same symmetry axis as the Homunculus nebula. This large outer shell is interacting with the surrounding medium, creating soft X-ray emission. We establish the shape and extent of the ghost shell in front of the southern Homunculus lobe and confirm that the NN condensation can best be modeled as a bowshock in the orbital/equatorial plane. Conclusions. The SHAPE modeling of the MUSE observations indicates that the kinematics of the outer ejecta measured with MUSE can be described by a spatially coherent structure, and this structure also correlates with the extended soft X-ray emission associated with the outer debris field. The ghost shell just outside the southern Homunculus lobe hints at a sequence of eruptions within the time frame of the Great Eruption from 1837-1858 or possibly a later shock/reverse shock velocity separation. Our 3D morpho-kinematic modeling and the MUSE observations constitute an invaluable dataset to be confronted with future radiation-hydrodynamics simulations. Such a comparison may shed light on the yet elusive physical mechanism responsible for eta Car-like eruptions.
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