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Register a new userThe enigmatic B[e]-star Henize 2-90: The non-spherical mass loss history from an analysis of forbidden lines
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(abridged) We study the optical spectrum of the exciting B[e] star Hen 2-90 based on new high-resolution observations that cover the innermost 2. Our investigation is splitted in two parts, a qualitative study of the presence of the numerous emission lines and the classification of their line profiles which indicate a circumstellar environment of high complexity, and a quantitative analysis of numerous forbidden lines, e.g. [OI], [OII], [OIII], [SII], [SIII], [ArIII], [ClII], [ClIII] and [NII]. We find a correlation between the different ionization states of the elements and the velocities derived from the line profiles: the highly ionized atoms have the highest outflow velocity while the neutral lines have the lowest outflow velocity. The recent HST image of Hen 2-90 reveals a bipolar, highly ionized region, a neutral disk-like structure and an intermediate region of moderate ionization. It seems that a non-spherical stellar wind model is a good option to explain the ionization and spatial distribution of the circumstellar material. We modelled the forbidden lines under the assumption of a non-spherically symmetric wind based on the HST image. We find that in order to fit the observed line luminosities, the mass flux, surface temperature, and terminal wind velocities need to be latitude dependent, which might be explained in terms of a rapidly rotating central star. A rotation speed of 75-80 % of the critical velocity has been derived. The total mass loss rate of the star was determined to be of order 3 10^{-5} M_sun/yr. Such a wind scenario and the fact that compared to solar abundances C, O, and N seem to be underabundant while S, Ar and Cl have solar abundances, might be explained in terms of a rapidly rotating post-AGB star.
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We present a study of the optical spectrum of the fascinating B[e] star Hen 2-90 based on new high-resolution observations taken with FEROS at the ESO 1.52m telescope in La Silla (Chile). The recent HST image of Hen 2-90 (Sahai et al. 2002) reveals a bipolar, highly ionized region, a neutral disk-like structure seen almost perfectly edge-on, and an intermediate region of moderate ionization. The slits of our observations cover the same innermost region of Hen 2-90 as the HST image, which allows us to combine the observations. Our spectra contain a huge amount of permitted and forbidden emission lines of atoms in different stages of ionization. In addition, the line wings deliver velocity information of the emitting region. We find correlation between the different ionization states of the elements and the velocities derived from the line profiles: the highly ionized atoms have the highest outflow velocity, while the neutral lines have the lowest. When combining the velocity information with the HST image of Hen 2-90, it seems that a non-spherical stellar wind model is a good option to explain the ionization and spatial distribution of the circumstellar material. Our modeling of the forbidden emission lines results in strong evidence for Hen 2-90 being a compact planetary nebulae that has undergone a superwind phase of high, non-spherical mass loss, most probably triggered by a central star that was rotating with about 80% of the critical velocity. We find a total mass loss rate during this superwind phase on the order of 3 x 10^(-5) M_sun/yr.
We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3 arcmin and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES).
Imaging and spectroscopy of the knots, clumps, and extended arcs in the complex ejecta of VY CMa confirm a record of high mass loss events over the past few hundred years. HST/STIS spectroscopy of numerous small knots close to the star allow us to measure their radial velocities from the strong K I emission and determine their separate motions, spatial orientations, and time since ejecta. Their ages concentrate around 70, 120, 200 and 250 years ago. A K I emission knot only 50 mas from the star ejected as recently as 1985 -- 1995 may coincide with an H2O maser. Comparison with VY CMas historic light curve from 1800 to the present, shows several knots with ejection times that correspond with extended periods of variability and deep minima. The similarity of this correspondence in VY CMa with the remarkable recent dimming of Betelgeuse and an outflow of gas is apparent. The evidence for similar outflows from the surface of a more typical red supergiant suggests that discrete ejections are more common and surface or convective activity is a major source of mass loss for red supergiants.
The rate of star formation both in the Galaxy and in external galaxies should be related to the physical properties of the molecular clouds from which stars form. This is expected for the starbursts found both in irregular galaxies and in some mergers. The dwarf galaxy Henize 2-10 is particularly interesting in this context as it shows a number of newly formed Super Star Clusters (SSCs) associated with a very rich molecular environment. We present a high angular resolution study of the molecular gas associated with the SSCs with the aim of deriving the physical properties of the parent molecular clouds. The final goal is to test the expectation that the formation of SSCs requires exceptionally dense and massive clouds. We have used the Submillimeter Array with an angular resolution of 1.9 X 1.3 to map the J=2-1 transition of CO in Henize 2-10. Supplementary measurements of HCN(J=1-0), 13CO(J=2-1) and millimeter continuum were obtained with the APEX, IRAM-30m and SEST single dish telescopes. Our single dish observations confirm the association of the newly formed SSCs in Henize 2-10 with dense molecular gas. Our interferometric observations resolve the CO(2-1) emission in several giant molecular clouds. Overall the molecular gas accounts for approximately half of the mass in the central regions of Henize 2-10. Although we find indications that the molecular clouds associated with the formation of SSCs in Henize 2-10 are massive and dense, the tracer we used (CO) and the linear resolution of our observations (60 X 80 pc) are still not adequate to test the expectation that exceptionally dense and massive cores are required for SSCs formation.
In the Sun, the two forbidden [OI] lines at 630 and 636 nm were previously found to provide discrepant oxygen abundances. aims: We investigate whether this discrepancy is peculiar to the Sun or whether it is also observed in other stars. method: We make use of high-resolution, high signal-to-noise ratio spectra of four dwarf to turn-off stars, five giant stars, and one sub-giant star observed with THEMIS, HARPS, and UVES to investigate the coherence of the two lines. results: The two lines provide oxygen abundances that are consistent, within observational errors, in all the giant stars examined by us. On the other hand, for the two dwarf stars for which a measurement was possible, for Procyon, and for the sub-giant star Capella, the 636 nm line provides systematically higher oxygen abundances, as already seen for the Sun. conclusions: The only two possible reasons for the discrepancy are a serious error in the oscillator strength of the NiI line blending the 630 nm line or the presence of an unknown blend in the 636 nm line, which makes the feature stronger. The CN lines blending the 636 nm line cannot be responsible for the discrepancy. The CaI autoionisation line, on the red wing of which the 636 nm line is formed, is not well modelled by our synthetic spectra. However, a better reproduction of this line would result in even higher abundances from the 636 nm, thus increasing the discrepancy.
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