The “Ghost Shell”: Discovery of the Forward Shock from Colliding Winds About Carinae

B. N. Dorland (Physics Dept., University of Maryland & Astrometry Dept., U.S. Naval Observatory, Washington DC)D. G. Currie (Physics Dept., University of Maryland & European Southern Observatory, Garching bei Muenchen, Germany)

A. Kaufer (European Southern Observatory, Santiago, Chile)

ABSTRACT We report the discovery of the “Ghost Shell” around the Luminous Blue Variable Carinae. The shell consists of a high-velocity, spatially extended emission feature lying in front of the Carinae Homunculus. Using data obtained with the VLT/UVES instrument, we have detected a narrow feature in velocity space moving at up to 500 km/sec faster than the Homunculus front wall. This shell covers the SE lobe of the Homunculus, and is observed to extend spatially beyond its boundaries. The Shell has been detected in emission for multiple allowed Balmer lines and in forbidden lines of [NII], [SII], and [ArIII]. The feature is also associated with a complex absorption structure in Ca H and K lines. We propose that the Ghost Shell lies outside the Homunculus and represents the forward shock between the fast stellar wind of the Great Eruption epoch and the older slow massive stellar wind. We present measurements of line strengths, spatial properties and velocity properties of the Shell and discuss the shape and proposed mechanism.

IV. Shape of the Ghost Shell

I. Carinae: Background

CarinaNebula

Carinae and the Homunculus observed with HST.

One of the most massive stars in the galaxy, Carinae is also one of the most enigmatic. It is thought to be an extreme member of the “Luminous Blue Variable” (LBV) class of evolved stars. According to the historical record, prior to 1842, its magnitude varied, with mv = +3 +/- 1. In 1842, it experienced a cataclysmic eruption that temporarily increased its magnitude to -1, making it one of the brightest stars in the sky. This “Great Eruption” resulted in, among other things, the creation of the “Homunculus”, a circumstellar nebula consisting of dust and gas ejected during the eruption. The maximum expansion velocity of the material composing the Homunculus is ~650 km/sec (Currie et al., 95, 96).

Carinae’s historical light curve.

1842 “Great Eruption”

1890 “Lesser Eruption”

UVES long slit observations of Carinae used to discover and analyze the Ghost Shell feature. “AOS” indicates Homunculus axis of symmetry.

Line of sightd = ~2.2 kpc

Axis of Symmetry

Equatorial PlanePlane of the Sky

i = 40 deg~17 arcsec~37 kAU~200 light-days~0.2 pc

NWlobe

SElobe

5”GhostShell(continuous)

GhostShell(disrupted)

UVES long slit observations of Ghost Shell for multiple emission species. Spectra have been aligned in velocity space for zero velocity reference wavelengths for H, H and [S II] 6731. “GS” indicates ghost shell feature, “FW” is homunculus front wall and “N” is background nebula.

Derived velocity data for Ghost Shell feature for multiple emission species. Velocities are along slit -7 (upper)

and axis of symmetry (lower) (i.e., between slits).

Proposed geometry of Ghost Shell, shown with respect to the current geometric model for the Homunculus. Ballistic (i.e., constant velocity) expansion is assumed.

UVES Channeled spectra for the Ghost Shell in the [N II] 6548.10 band. NW is up, SE down. Central star is visible in center for all velocities. This sequence demonstrates the spherical nature of the shell in this region.

-850 km/sec -800 km/sec -750 km/sec

-700 km/sec -650 km/sec -600 km/sec

Fe I (16 lines)

II. Ghost Shell Observations: Emission

V. Conclusions

Line of sight

d = ~2.2 kpc

Axis of Symmetry

Equatorial PlanePlane of the Sky

i = 40 deg

vDoppler = -600 km/sec

vradial = 650 km/sec

vradial ~ 200 km/sec

vDoppler ~ -150 km/sec

Rewind the clock-> 1842 “Great Eruption”Currie, Dowling, Shaya et al. (1996)

~17 arcsec~37 kAU~200 light-days~0.2 pc

NWlobe

SElobe

Spikes~2000 km/sec

5”

Homunculus features and scale

6500 6600

6563H

6548[N II]

6583[N II]

?

12 arcsec

Absorption Spectrum: Slit -3

Ca II H

0-500-1000

3960 3970

3760 4000

H- 7-2H- 8-2H- 9-2H- 10-2H- 11-2

Ca II HCa II K

He I 8-2

In December 1999 and January 2000, the European Southern Observatory’s (ESO) Ultraviolet and Visible Echelle Spectrometer (UVES) and the Very Large Telescope (VLT) were used to observe a grid of slits that covered Carinae’s Homunculus.

The data were collected in a spectral range of 3100-6800 angstroms, with a spectral resolution of R = 70,000-100,000 and under seeing conditions of ~ 0.6 arcsec.

Initial analysis of the data revealed a narrow intrinsic emission feature (FWHM ~ 35 km/sec, resolved) in slit -8 for both [NII] and H with a centroid velocity of -825 km/sec, hundreds of km/sec faster than the Homunculus front wall.

6500

Further analysis detected this feature, named the “Ghost Shell”, in at least six other emission lines.

Velocity correlation between the various lines is excellent, as shown both by lining velocity space images up next to one another (l.) and by plotting feature velocity centroids as a function of position along slit (r. lower) or slit number (r. upper).

In addition to the emission lines, we have also observed absorption lines in Ca H and K that correspond to the Ghost Shell. We see in the Ca H absorption line (upper) that the Ghost Shell absorption lies outside the Homunculus in velocity space along the axis of symmetry. It has a complex structure, as does the front wall of the Homunculus.

These Ca features appear to correspond with anomalous Ca features noted by Davidson et al. (2001). In the lower plot, we show the excellent agreement between our emission line velocity measurements (for the [NII] 6548.10 line) and Davidson’s absorption data (error bars indicate our measurement error in measuring Davidson’s velocities).

III. Ghost Shell Observations: Absorption

Plots of the velocity as a function of position (both between slit and cross slit) suggest the Ghost Shell is roughly spherical in shape, given an assumption of ballistic (i.e., unaccelerated) motion. In order to better visualize this, we created “channeled spectra” by creating a 3-D cube using the grid data for the first two dimensions and the velocity data for the third. We then take iso-velocity cross sections of the resultant data cube. We would expect a uniformly expanding sphere to produce circular cross sections; the results for the Ghost Shell (shown at right) are very similar, suggesting a quasi-spherical shape, at least over the SE lobe region. Given the assumption of ballistic motion,

an accepted approximation for the Homunculus, we can transform velocities into displacements. This allows us to create a spatial cross section, shown on the left, that shows the positions of the Homunculus features and the Ghost Shell, approximately along the axis of symmetry.

We note that the Ghost Shell structure is continuous only in the SE lobe region. Portions of it appear to extend over the entire NW lobe. We believe this section of the Ghost Shell has been disrupted by the passage of higher-velocity ejecta.

Identified Ghost Shell emission lines. Second column indicates reference wavelength in air, third and fourth columns indicate which slits lines were observed in, and fifth column is relative line strength, with H = 1.00.

17 arcsec

12 arcsec

Central star

Ghost shell

Ghost shell Homunculus Front Wall

~17 arcsec

We have discovered and investigated a shell about the homunculus which has the appearance of a high velocity (1000 km/sec) shock wave. However, unlike the high velocity shocks found in young supernova remnants, the gas has a high velocity, moving toward us at 850 km/sec, and the spectrum appears very similar to the YSO shocks of about 150 km/sec (see table). However, additional observational data and numerical simulations of high velocity shocks in dense media are urgently needed to understand the physics of this remarkable phenomena.

VI. References

Some of these results are discussed in the following paper:• Currie, D.G., Dorland, B.N., and Kaufer, A., 2002, A&A, 389, L65

Other references:• Currie, D.G., Dowling, D., Shaya, E., 1995, Proc. ESO Workshop “The Role of Dust in the Formation of Stars”, 89

• Currie, D.G., Dowling, D., Shaya, E., et al., 1996, AJ, 112, 115• Davidson, K., Smith, N., Gull, T.R., et al., 2001, ApJ, 121, 1569