The enigmatic B[e]-star Hen 2-90

The nature of the very interesting object Hen 2-90 was longtime debated in the literature. It was first classified as a planetary nebula. Later on, based on images obtained with the Hubble Space Telescope (HST), the presence of a nebula bisected by an almost edge-on seen disk and a bipolar jet with uniformly spaced knots were discovered (see the following figures taken from Sahai & Nyman, 2000, and Sahai et al., 2002). The dynamical stability found for the jets and knots makes Hen 2-90 definitely a unique object.



The characteristics (nebula, disk, jets, knots) could point to a compact planetary nebula nature of Hen 2-90. On the other hand, the presence of the disk and the jets could also indicate a binary nature of the system. Furthermore, its near-infrared colors agree with those of symbiotic objects, i.e., a binary consisting of a cool giant and a hot compact object surrounded by an accretion disk. These completely different interpretations of Hen 2-90's characteristics in terms of either a symbiotic object or a compact planetary nebula made us curious to disentangle the real nature of Hen 2-90.

We obtained high- and low-resolution optical spectoscopic observations of Hen 2-90 in June 2000 with the Fiber-fed Extended Range Optical Spectrograph (FEROS) and with the Boller & Chivens (B&C) spectrograph, respectively, at the 1.52-m telescope at the European Southern Observatory (ESO) in La Silla (Chile). The spectra display many permitted and forbidden emission lines. Most of them come from singly or doubly ionized elements. The line profiles could be grouped into different categories: (i) double-peaked profiles, (ii) broad single-peaked profiles, (iii) narrow single-peaked profiles, and (iv) profiles with a "shoulder".

FEROS uses the fibre technique with round fibers covering 2 arcsec. Comparison with the HST image thus reveals that we observed a substantial amount of the nebulosity and the disk of Hen 2-90. The size of the fiber is shown as the white circle in the top panel of the following figure. The variety of elements in different ionization stages together with the variety in line profiles hint towards the simultaneous presence of cool and hot gas being subject to different kinematics.


The nature of the circumstellar material: nebula or wind ?

The color coding of the HST image allows us to distinguish between a high-ionized (in green) polar region (i.e. in the directions of the jets) traced by the forbidden line of doubly ionized oxygen, and an intermediate region (in red) traced by the forbidden line of singly ionized nitrogen. The dark lane of the almost edge-on seen disk must then be the location of the cool material like the neutral oxygen. The obvious change in ionization degree with latitude means that we see a drop in temperature from the pole to the equator. This decrease in temperature is most probably linked to an increase in density in the circumstellar material, else it is not possible to explain the simultaneous presence of high- and low-ionized gas at equal distances from the presumably quite hot central star. Inspecting the widths of the forbidden emission lines that arise within the different regions, we found that the lines from the polar regions show the highest velocity, while those from the disk region display the smallest velocity component. Such a scenario is incompatible with the nebula approximation usually applied to planetary nebulae. Instead, it seems likely that the central star of Hen 2-90 drives a non-spherically symmetric wind that has the highest temperature and velocity but lowest density in polar direction, while in equatorial direction it has the highest density but lowest temperature and velocity. Such a secnario is well-known from rapidly rotating stars. From the ratio of the outflow velocities in polar and equatorial directions, we find that Hen 2-90 would have to rotate with 75-80% of its critical velocity. Armed with the latitude-dependent wind scenario, we could reproduce the observed line luminosities of the variety of forbidden emission lines detected in the spectrum of Hen 2-90. The kinematical wind model also allows to explain qualitatively the different types of line profiles so that we are confident that the suggested model is reasonable.

The nature of Hen 2-90: symbiotic object or compact planetary nebula ?

The HST image of Hen 2-90 shows the presence of a nebula bisected by a disk, with a highly collimated and bipolar jet with several pairs of knots on both sides. This scenario could be explained assuming that Hen 2-90 is either a compact planetary nebula, where the wind asymmetries started during the asymptotic giant branch (AGB) phase, or a binary system, where the jets are caused by an accretion disk.

In case of a symbiotic nature, we would expect to see hints for both the hot and the cool component. The hot component is usually revealed by emission lines of ionized Helium (He II) and other elements with high ionization potential, while the cool component usually displays absorption signatures of molecular bands like titanium oxide (TiO). Our FEROS spectra are of excellent quality; nevertheless, we do not find any indication for either absorption bands of TiO nor for emission of He II or any other highly-ionized element, so that we can conclude that these features are definitely not present. Also, we do not see any indication for ongoing accretion. Hence, a symbiotic nature of Hen 2-90 can be excluded. We thus conclude that Hen 2-90 is most probably a compact planetary nebula.

Unsolved problems and additional curiosities

Although our description with a latitude dependent wind of a rapidly rotating central star delivers reasonable results, we are left with a number of unsolved questions:

During our modeling of the individual forbidden emission lines we found a depletion of nitrogen (N) and oxygen (O) by a factor of 2 and 3.3, respectively, while other elements like sulfur (S), argonium (Ar), and chlorine (Cl) have normal, i.e. solar, abundances. Carbon (C) appears to be just at the detection limit, and we tentatively derive its abundance to less than 0.6 solar. While a depletion of C and O is a natural outcome from stellar evolution, we have no explanation for the simultaneous depletion in N. There are, however, some B-type post-AGB stars in the halo of our Galaxy which also show this behaviour; but the origin of this anomaly is still poorly understood.

Another unsolved problem is the formation of the highly collimated jets and knots on constant time intervals. Such a behavior has never been observed before making Hen 2-90 a unique object. Usually, the formation of jets is ascribed to the presence of an accretion disk, probably triggered by an interacting binary. While we have excluded a symbiotic nature of Hen 2-90, we cannot definitely exclude a binary nature of the central object. In that respect it is also important to note that the postulated rapid rotation of the central star as the cause of the latitude dependent wind structure and disk formation, is usually not observed in compact planetary nebulae. A possible explanation could be that the central object of Hen 2-90 is the remnant of a binary merger, during which the star spun up to rotation velocities close to critical.

Only follow-up observations and investigations can help to understand the curious characteristics of this highly interesting object.

Related publications :

  • Kraus, Borges Fernandes, de Araújo, & Lamers, 2005, A&A, 441, 289

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    Last modified 2.2.2011