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 sophis­ticated 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 Homun­culus 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. Their pro- jected separation is about 110 degrees, %OXHOREH very close to the apparent opening angle %OXHqKROHr of the wind–wind bowshock produced by the massive interacting winds of η Car when the stars are at apastron (Madura et al., 2012). The two trenches (Figure 4)

%OXHSURWUXVLRQ have never before been identified and are similar in angular extent and position rela- %OXHWUHQFK %OXHOREH tive to the central binary. 5HGSURWUXVLRQ Independently obtained and apparently 5HGOREH disconnected results that turn out to have a strong correlation hint towards a ,UUHJXODULWLHV causal relation between the interacting winds and the features of the Homunculus 5HGqKROHr we have identified in our analysis. We note that the 3D reconstruction of the %OXHSURWUXVLRQ Homunculus Nebula was obtained without assuming the orientation of the orbit of 5HGWUHQFK the central binary. The apastron direction Figure 5. A comparison is shown between the open- derived by Madura et al. (2013) projects ing angle of the wind–wind interaction region of the central binary star (red–blue) and the angular distance Figure 4. Two renderings of the 3D surface structure between the two protrusions. Further- between the newly found protrusions in the Homun- of the Homunculus model that emphasises the small- more, the angular size of the trenches in culus (grey). The red–blue colours depict density scale structure (adopted from Steffen et al., 2014). The structures as derived by hydrodynamic simulations in view from Earth is shown in the upper image, whereas the polar regions is very similar to the the orbital plane of the binary (upper) and perpendicu- the lower one is a view from the opposite direction. angular extent of the protrusions, and lar to it (lower), from Madura et al. (2013). The diame- The lobes are denoted according to their actual red or their orientations are also centred on the ter of the (grey) Homunculus section is about 6 arc- blue spectral line shift as viewed from Earth. (A 3D ver- apastron direction. seconds, while the interacting winds extend to only sion is available in the online Messenger.) about 1.5 arcseconds in these images. In Figure 5 we compare the orientation al. (2008) from maps of the near-infrared of the two projections to 3D models of [Fe II] 1.257 μm line. Looking more closely, the massive interacting wind derived by tion of the orbital plane, as illustrated in these “holes” are off-centre by roughly Madura et al. (2013). The model is derived Figure 6. We therefore tentatively predict eight degrees. Each polar cap is gouged from the spatial structure seen in [Fe III] that the orbital plane might be tilted with by a trench, each of which is positioned from detailed maps made with STIS respect to the equatorial plane of the with approximate point-symmetry, not (Gull et al., 2009). Multiple distorted shells Homunculus Nebula by 20 degrees. Such mirror-imaged with respect to the central have been created by wind–wind inter­ a misalignment between the orbital plane star. There are also red and blue protru- action during the current and previous and axis of the Homunculus has been sions extending from the lobes near the orbits of the binary system; because of suggested earlier, based on the analysis mid-plane of the nebula. One protrusion the high eccentricity (e ≈ 0.9) the stars of spectroscopic observations and is seen on the near lobe to the right spend most of their orbits near apastron. hydrodynamical simulations (Groh et al., (west) and another on the far lobe point- The net result is a large cavity carved 2010, and references therein). However ing to the top (north). Added to Figure 3 out in the apastron direction of the sec- the models by Madura et al. (2013) sug- are three lines that lie in the equatorial ondary star; the opening angle of this gest that the orbital plane appears to plane: projections of the red and blue wind–wind interaction cone is strikingly be closely aligned, within 10 degrees, of protrusions onto the equatorial surface similar to the azimuthal distance between the Homunculus axis of symmetry. The and the apastron direction of the binary. the protrusions (see Figure 5). Obtained two lobes of the Homunculus appear The latter was derived by Madura et al. from a completely independent investiga- to be about 8 degrees out of alignment. (2013) based on 3D models of the ex­­ tion, this comparison suggests a causal tended winds mapped deep within the relation between the wind–wind interac- Based on the kinematics derived from Homunculus by the Space Telescope tion and the protrusions. molecular hydrogen lines observed with Imaging Spectrograph (STIS) on board X-shooter and using the Shape software, HST (Gull et al., 2009). Based on this evidence for a direct influ- several features have been revealed that ence of the interacting stellar winds on are highly aligned with the orientation We initially thought that the protrusions the small-scale structure of the Homun- of the line of centres of the η Car binary near the equatorial region, at latitudes culus, we speculate that the line connect- system. The azimuthal angular extent and between about 10 degrees to 20 degrees ing the two protrusions defines the direc- location of the features coincides with

28 The Messenger 158 – December 2014 Orbit of Figure 6. Schematic diagram of the Acknowledgements primary star proposed inclination of the orbit of the secondary star with respect to the W. S. acknowledges support for this project by equatorial plane of the Homunculus. UNAM-PAPIIT grant 101014.

References

Damineli, A. et al. 2008, MNRAS, 386, 2330 Equator ~20˚ Davidson, K. & Humphreys, R. M. (eds.) 2012, Eta Carinae and the Supernova Impostors, Astro­ Homunculus physics and Space Science Library, (New York: Orbit of Springer), 384 secondary star Groh, J. H. et al. 2010, A&A, 517, A9 Gull, T. R. et al. 2009, MNRAS, 396, 1308 Ishibashi, K. et al. 2003, AJ, 125, 3222 the opening angle of the conical cavity Interactive and 3D printable model Madura, T. I. 2012, MNRAS, 420, 2064 Madura, T. I. et al. 2013, MNRAS, 436, 3820 produced by the interaction of the stellar Steffen, W. et al. 2011, IEEE Transactions on winds of the primary and secondary, We have created an interactive version of Visualization and Computer Graphics, 17, 4, 454 ­previously derived by independent hydro- the 3D model of the Homunculus that Steffen, W. et al. 2014, MNRAS, 442, 3316 dynamic modelling (Madura et al., 2013). can be accessed via Figure 5 of the origi- Teodoro, M. et al. 2008, MNRAS, 287, 564 We have therefore shown that the inter- nal publication in Steffen et al. (2014) in acting winds of the central binary have PDF format. This 3D graphic is also avail- Links had a direct impact on the surrounding able, as Figure 4, in this article. Further- 1 Homunculus Nebulae, either briefly more, a 3D printable version in STL SHAPE software package: http://www.astrosen.unam.mx/shape ­during the 1840’s eruption, or slowly dur- ­format is available as supplementary 2 Access to file for 3D printer version:http://mnras. 2 ing the many orbital periods that have material to the publication . oxfordjournals.org/content/suppl/2014/06/30/ passed since the eruption. stu1088.DC1/supp.zip

Colour composite image of the star-forming complex NGC 3372, including η Carinae (lower left of centre). This image was made with the MPG/ESO 2.2-metre telescope and the Wide Field Imager by combining images in broad bands (U, B, V and R) and narrow bands (Hα and [S II]). The field of view is 33 arc- minutes square (north up, east left) and the clusters Trumpler 14 and Collinder 229 can be seen to the northwest and southeast of η Car respectively. See eso0905 for more details.

The Messenger 158 – December 2014 29