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