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Astron. Astrophys. 320, 568-574 (1997)

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5. Structure and environment

We conclude that the nebula of HR Car is of bipolar structure and assume that the radial expansion velocity is no larger than the full velocity split and no smaller than 1/2 of the velocity split, or 75 km s [FORMULA] km s-1. The northwestern and southeastern lobes have angular sizes of [FORMULA] and [FORMULA] along the slit, respectively. For a distance of 5 kpc, the linear sizes would be 0.63 and 0.67 pc. Assuming no additional acceleration, a lower limit on the dynamic age of the bipolar nebula is [FORMULA] yr.

The bipolar nebula of HR Car show intriguing similarities with the bipolar Homunculus nebula around the famous LBV [FORMULA] Car. At a distance of 2.5 kpc, each lobe of the Homunculus has a diameter of [FORMULA] pc. This is a factor of 6 smaller than the lobes of HR Car. Therefore, it is very likely that the nebula around HR Car is an older version of the Homunculus around [FORMULA] Car.

The gaseous environment of HR Car can be seen in the H [FORMULA] images in Figs. 1 and 2. A large funnel-shaped nebula is visible to the northwest of HR Car. This nebula is very faint. Only the brightest parts are barely visible on the ESO R plate in the Southern Sky Atlas. The axis of the funnel is roughly aligned with the the polar axis of the bipolar nebula around HR Car. This morphology suggests that HR Car is responsible for the ionization and shaping of this nebula.

Is this funnel-shaped nebula ejected by HR Car or an interstellar bubble blown by HR Car? At a distance of 5 kpc, the large shell structure would be located at 3 to 4.5 pc from HR Car. The ejecta of an LBV would need 26000 years to reach this distance from the star (assuming [FORMULA] km s-1). This time interval is comparable to the lifetime of the LBV phase. Assuming [FORMULA] km s-1, a typical intermediate velocity of the shell from a LBV ejecta (García-Segura et al. 1996), the filament at around 4 pc would be reached within 16000 yr. This would imply the possibility that this structure could be a remnant of an older ejecta from HR Car. If so, HR Car may be an already older LBV, which is consistent with its position in the HR diagram. An older age is implied by HR Car's being situated significantly beneath the Humphreys-Davidson limit (Humphreys & Davidson 1994).

Another possible scenario to connect this outer nebulosity with HR Car is to identify it as a wind-blown bubble. Fast stellar wind of massive stars in their main-sequence phase will sweep up ambient medium and form "interstellar bubbles" (Weaver et al. 1977). The dynamic age of such a bubble is [FORMULA]), with R being the bubble radius and [FORMULA] its expansion velocity. The parameter [FORMULA] is 0.6 for an energy-conserving bubble in a homogeneous medium (Weaver et al. 1977) or 0.5 for a momentum-conserving bubble (Steigman et al. 1975). Assuming a typical wind-blown bubble expansion velocity of 20 km s-1, the dynamical age of this bubble (with the northern filaments being its border) would be between 1 and [FORMULA] yr. With HR Car being already in the LBV phase a wind-blown bubble created in the main sequence state must be older than the [FORMULA] yr we find. If the northwestern nebulosity is connected to HR Car a wind-blown bubble scenario is not likely.

A net-like internal structure of this feature can very well be seen in Fig.2, fitting to the scenario of an expanding shell structure. We note that as seen in Fig. 2 this nebulosity shows an additional [FORMULA] e.g. 2.9 pc long feature reaching towards the north-west. One may link this elongated cone to a blow out structure. To prove this and explain the origin of the north-western filamentary nebula further spectroscopic and kinematic investigations are needed.

Our echelle observations detect a background H [FORMULA] emission over the entire slit length ([FORMULA]) at both slit positions. The velocity of this gas is [FORMULA] km s-1 (see Fig. 4 and Fig. 5, the dashed line). This background H II component has a velocity FWHM of 41 km s-1 (after removing the instrumental broadening of FWHM of 14 km s-1). This velocity width, higher than the thermal broadening for a 104 K gas, indicates gas motion; however, there is no evidence for an expanding shell, since no line-split is observed. This could be caused by the presence of multiple unresolved velocity components or a large turbulence. As discussed in 4.1 this may be due to gas in the Carina HII Complex that is in line of sight to HR Car, or indicates extended diffuse local emission around HR Car.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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