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The Eta Carinae Homunculus in Full 3D with X-Shooter and Shape

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The Eta Carinae Homunculus in Full 3D with X-Shooter and Shape Astronomical Science The Eta Carinae Homunculus in Full 3D with X-shooter and Shape Wolfgang Steffen1 its effect on the surrounding gas and is ideally suited for mapping the Homun- Mairan Teodoro2 dust, including the famous Homunculus culus. The 0.4 by 11 arcsecond slit pro- Thomas I. Madura2 Nebula. A comprehensive overall review vided seeing-limited spatial resolution José H. Groh3 by many authors on different aspects with spectral resolving power of 11 300, Theodore R. Gull2 of η Car and its surroundings can be which is well matched to the approxi- Andrea Mehner4 found in Davidson & Humphreys (2012). mately ten-arcsecond-wide bipolar struc- Michael F. Corcoran2 ture. Since the structure expands at Augusto Damineli5 In recent work (Steffen et al., 2014), velocities up to 650 km s–1 along the line Kenji Hamaguchi6 based on spectroscopic observations of sight, image slices with 50 km s–1 with X-shooter on the Very Large Tele- separation provided detailed maps of the scope (VLT), we found the first strong structure along the line of sight. While 1 Instituto de Astronomía, UNAM, evidence for a direct impact of the binary the spectral range of the data extended Ensenada, Mexico system on the structure of the bipolar from 2999 Å to 24 790 Å, for this par- 2 Goddard Space Flight Center, Homunculus Nebula that was ejected in ticular work we focussed on a single Greenbelt, USA the giant eruption around 1840. Here emission line of molecular hydrogen, the 3 Geneva Observatory, Geneva University, we briefly describe the motivation, ob­­ v = 1-0 S(1) line at 2.12125 μm. Sauverny, Switzerland servations and results of this work. It is 4 ESO particularly interesting how the rather The Homunculus was mapped with a 5 Instituto de Astronomía, Geofísica e simple geometric results connect to total of 92 dithered positions orienting the Cîencias Atmosféricas, Universidade de sophis ticated theoretical simulations of slit perpendicular to the projected bipolar São Paulo, Brazil the interacting winds of the central binary axis (position angle [PA] = –41 degrees). 6 Department of Physics, University of (Madura et al., 2013). To provide a suitable dynamic range for Maryland, Baltimore, USA each slit position, the exposure times Owing to the large difference in bright- ranged from 30 seconds in the outer ness and closeness to the primary, regions to 0.067 seconds over the central Massive stars like Eta Carinae are the existence of the secondary can only star. Multiple images at each position extremely rare in comparison to stars be inferred indirectly, mainly through were then combined to yield a total expo- such as the Sun, and currently we the 5.5-year periodicity in the X-ray band sure time from 30 seconds in the central know of only a handful of stars with and spectroscopic changes in the ultra- region to 150 seconds in the outer lobes. masses of more than 100 MA in the violet, optical and infrared bands (Damineli Milky Way. Such massive stars were et al., 2008). Finding direct evidence of After standard processing for individual much more frequent in the early history the binary interaction in the Homunculus spectra, a datacube was generated on of the Universe and had a huge impact Nebula would therefore shed some light a regular position–velocity (P–V) grid on its evolution. Even among this elite on the eruption process itself and would interpolating between actual slit posi- club, η Car is outstanding, in particular help to further constrain the orientation tions. Figure 1 shows an optical Hubble because of its giant eruption around of the highly elongated orbits. The fact Space Telescope (HST) image of the 1840 that produced the beautiful bipolar that there are two nebulae, the Little Homunculus with the observed slit posi- nebula now known as the Homunculus. Homunculus (Ishibashi et al., 2003) tions in grey and the interpolated slit In this study, we used detailed spatio- located within the Homunculus, and the positions for modelling in red. For the kinematic information obtained from fast-moving ejecta outside the Homun- 3D modelling process, 16 slit positions X-shooter spectra to reconstruct the 3D culus, both exhibiting the same elongated were selected from the combined data- structure of the Homunculus. The small- shape, provides hints that the binary sys- cube based on significant structural scale features suggest that the central tem played a role in shaping the eruptions. changes. These 16 P–V images are dis- massive binary played a significant role played in Figure 2 together with P–V in shaping the Homunculus. To achieve these goals, the structure of images synthesised from the model. the Homunculus needed to be derived in more detail than that provided by the Introduction available axi-symmetric models. Spatially 3D modelling resolved kinematic data with full coverage In August 2014 the stars of the η Car were needed in the infrared, since in the The 3D modelling was done with the binary system were at their closest visible spectrum the Homunculus is opti- virtual astrophysical modelling laboratory approach. The observable changes in cally thick, greatly obscuring the far sides Shape (version 5, Steffen et al., 2011)1. the weeks around periastron are rather of the bipolar lobes. The general assumption for the recon- dramatic, because of the highly eccentric struction of the position information orbit. Therefore, many groups had been along the line of sight is that the nebular preparing to observe η Car, looking for Observations material was ejected in a short interval changes that might help us learn more of time in the relatively distant past and about this complex system of high-mass The X-shooter spectrograph mounted on is expanding homologously (i.e., the stars and interacting stellar winds, and the VLT’s Kueyen Unit Telescope (UT2) position vector of each volume element 26 The Messenger 158 – December 2014 - –8.87 Slit 7 –8.03 Slit 11 –7.61 Slit 13 –7.19 Slit 15 $ –6.14 Slit 20 –5.51 Slit 23 –4.88 Slit 26 –4.04 Slit 30 BRDBNMC @Q 8 6 l –1.94 Slit 40 +0.16 Slit 50 +1.63 Slit 57 +3.31 Slit 65 Blue protrusion Red protrusion l l ) 67@QBRDBNMC 93 6. +5.41 Slit 75 +6.25 Slit 79 +8.14 Slit 88 +8.56 Slit 90 Figure 1. Observed X-shooter slit positions (hori- second rc zontal rectangles) superimposed on an optical 3 Observation (a Hubble Space Telescope image of the Homunculus .9 Nebula. The spectra were combined to generate a –6 continuous datacube. The red horizontal lines show sition the positions at which position–velocity images Po Model were extracted from the datacube for modelling (see –750 0 +800 Steffen et al. [2014] for more detail). Radial velocity (km s–1) Figure 2. The 16 observed position–velocity images that have been used for 3D modelling are shown (rows 1, 3, 5, 7), together with the corresponding is proportional to its velocity vector). tion in order to allow the matter to sort model position–velocity images (rows 2, 4, 6 and 8) Physically, such an expansion pattern is itself out according to its velocity. beneath (from Steffen et al., 2014). The slit numbers expected from an explosion or eruption and offset along the Homunculus long axis (Figure 1) in which the gas is accelerated very If the expansion can be assumed to be are indicated. quickly and then expands with constant homologous and there is a good degree speed radially outwards. Furthermore, of axial symmetry, then an accurate field are then defined. Finally, the software the process of ejection must be short mapping can be derived between posi- computes spatial and position–velocity compared to the time that has elapsed tion and observed velocity along the line images for the synthetic slit positions that between the eruption and the observa- of sight. The massive, dense eruption that correspond to the observations. The produced the Homunculus is expected results can then be compared directly to expand nearly homologously because with the observations. The differences O@RSQNM 1DCOQNSQTRHNM of its high density compared to the local between the observations and model CHQDBSHNM circumstellar environment, which is prob- output are then iteratively corrected until ably a hollow cavity blown out by the the result satisfactorily reproduces the earlier stellar winds. Previous work and observations. In Figure 2, observed and ¦ our reconstruction are consistent with only synthetic position–velocity images are very minor deviations from large-scale shown together, which allows an estimate ¦ homologous expansion. of the precision of the reconstructed geometry. The reconstruction was done in the Shape software´s interactive mesh recon- !KTDOQNSQTRHNM struction mode. The general procedure Results for such reconstructions is to interactively generate a 3D mesh shell that represents The resultant 3D mesh structure, shown the observed gas shell volume (Figure 3). in Figure 3, provides significant details Figure 3. The 3D model mesh of the Homun culus as The detailed structure of this mesh is of the well-known bipolar structure seen from Earth. The direction of binary apastron obtained by deforming an initially spherical seen at optical wavelengths, as imaged and its angular relation to the newly discovered pro- trusions are marked with dashed lines (adapted from object using a variety of “modifier” tools. in Figure 1. At the poles of each lobe Steffen et al., 2014). An emissivity distribution and a velocity are holes, previously found by Teodoro et The Messenger 158 – December 2014 27 Astronomical Science Steffen W. et al., The Eta Carinae Homunculus in Full 3D with X-shooter and Shape might be due to a ring-like structure, but 5HGOREH 5HGSURWUXVLRQ the modelling showed that they are quite localised, one on each lobe.
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