N II 5668-5712, a New Class of Spectral Features in Eta Carinae

We report on the N II 5668-5712 emission and absorption lines in the spectrum of Eta Carinae. Spectral lines of the stellar wind regions can be classified into four physically distinct categories: 1) low-excitation emission such as H I and Fe II, 2) higher excitation He I features, 3) the N II lines discussed in this paper, and 4) He II emission. These categories have different combinations of radial velocity behavior, excitation processes, and dependences on the secondary star. The N II lines are the only known features that originate in "normal" undisturbed zones of the primary wind but depend primarily on the location of the hot secondary star. N II probably excludes some proposed models, such as those where He I lines originate in the secondary star's wind or in an accretion disk.Comment: 4 figures, 1 tabl

Multiplicity of tight ultra low mass systems

A number of scenarios for the formation of brown dwarfs are currently being discussed. Binary properties constrain these scenarios. While many imaging surveys were carried out to find visuell binaries, few surveys were conducted to find short-period binaries. By deriving the frequency of short-period binary brown dwarfs, the importance of momentum transfer and tidal interactions for the formation of binary brown dwarfs can be studied. A radial velocity survey is the most efficient way to find short-period brown dwarf binaries. In this work, high-resolution spectra of 27 very-low mass stars and brown dwarfs were analyzed. The data was taken with UVES at the VLT (Paranal, Chile) in the years 2001 to 2004. Parts of this data were already analyzed by Guenther and Wuchterl (2003). For the work presented here, all data from the years 2001 to 2004 was used. Additionally, the accuracy of the radial velocity measurement was improved. Two spectroscopic binaries were confirmed. It was shown that there is no additional binary with a period of ! 40 days in this sample. By improving the accuracy of the measurements and increasing the number of spectra, the binary candidate, LHS 292, turned out not to be a binary. Thus, the binary frequency is 7.4 ± 1.4%. This frequency supports the solar-like formation scenario of very low-mass stars and brown dwarfs and it shows that momentum transfer during the formation is not of great importance. To study the effects of magnetic fields on the atmospheres of brown dwarfs, I observed the brown dwarf LP 944-20 with EFOSC2 on the 3.6 m telescope in La Silla, Chile. LP 944-20 is an ideal object for this purpose. The magnetic field strength at the surface of LP 944-20 may be nearly 1 kG, even in quiescence. There are no significant variations, neither in the equivalent widths of the H! and Na D emission lines nor the temperature. It can be concluded that LP 944-20 has a very homogeneous atmosphere. It is highly unlikely that spots or other prominent surface features exist on this object

Unexpected Ionization Structure in Eta Carinae's "Weigelt Knots"

The Weigelt knots, dense slow-moving ejecta near η Carinae, are mysterious in structure as well as in origin. Using spatially dithered spectrograms obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS), we have partially resolved the ionization zones of one knot. Contrary to simple models, higher ionization levels occur on the outer side, i.e., farther from the star. They cannot represent a bow shock, and no satisfying explanation is yet available—though we sketch one qualitative possibility. STIS spectrograms provide far more reliable spatial measurements of the Weigelt knots than HST images do, and this technique can also be applied to the knots' proper motion problem. Our spatial measurement accuracy is about 10 mas, corresponding to a projected linear scale of the order of 30 AU, which is appreciably smaller than the size of each Weigelt knot

Critical Differences and Clues in Eta Car's 2009 Event

We monitored Eta Carinae with HST WFPC2 and Gemini GMOS throughout the 2009 spectroscopic event, which was expected to differ from its predecessor in 2003 (Davidson et al. 2005). Here we report major observed differences between events, and their implications. Some of these results were quite unexpected. (1) The UV brightness minimum was much deeper in 2009. This suggests that physical conditions in the early stages of an event depend on different parameters than the "normal" inter-event wind. Extra mass ejection from the primary star is one possible cause. (2) The expected He II 4687 brightness maximum was followed several weeks later by another. We explain why this fact, and the timing of the 4687 maxima, strongly support a "shock breakup" hypothesis for X-ray and 4687 behavior as proposed 5-10 years ago. (3) We observed a polar view of the star via light reflected by dust in the Homunculus nebula. Surprisingly, at that location the variations of emission-line brightness and Doppler velocities closely resembled a direct view of the star; which should not have been true for any phenomena related to the orbit. This result casts very serious doubt on all the proposed velocity interpretations that depend on the secondary star's orbital motion. (4) Latitude-dependent variations of H I, He I and Fe II features reveal aspects of wind behavior during the event. In addition, we discuss implications of the observations for several crucial unsolved problems.Comment: 45 pages, 9 figures, submitted to Ap

Mid-Cycle Changes in Eta Carinae

In late 2006, ground-based photometry of $\eta$ Car plus the Homunculus showed an unexpected decrease in its integrated apparent brightness, an apparent reversal of its long-term brightening. Subsequent HST/WFPC2 photometry of the central star in the near-UV showed that this was not a simple reversal. This multi-wavelength photometry did not support increased extinction by dust as the explanation for the decrease in brightness. A spectrum obtained with GMOS on the Gemini-South telescope, revealed subtle changes mid-way in $\eta$ Car's 5.5 yr spectroscopic cycle 0when compared with HST/STIS spectra at the same phase in the cycle. At mid-cycle the secondary star is 20--30 AU from the primary. We suggest that the spectroscopic changes are consistent with fluctuations in the density and velocity of the primary star's wind, unrelated to the 5.5 yr cycle but possibly related to its latitude-dependent morphology. We also discuss subtle effects that must be taken into account when comparing ground-based and HST/STIS spectra.Comment: 34 pages, 9 Figure

Critical Differences and Clues in Eta Car\u27s 2009 Event

We monitored Eta Carinae with the Hubble Space Telescope WFPC2 and Gemini GMOS throughout the 2009 spectroscopic event, which was expected to differ from its predecessor in 2003. Here we report major observed differences between events and their implications. Some of these results were quite unexpected. (1) The UV brightness minimum was much deeper in 2009. This suggests that physical conditions in the early stages of an event depend on different parameters than the normal inter-event wind. Extra mass ejection from the primary star is one possible cause. (2) The expected He II λ4687 brightness maximum was followed several weeks later by another. We explain why this fact and the timing of the λ4687 maxima strongly support a shock breakup hypothesis for X-ray and λ4687 behavior as proposed 5-10 years ago. (3) We observed a polar view of the star via light reflected by dust in the Homunculus nebula. Surprisingly, at that location, the variations of emission-line brightness and Doppler velocities closely resembled a direct view of the star, which should not have been true for any phenomena related to the orbit. This result casts very serious doubt on all the proposed velocity interpretations that depend on the secondary star\u27s orbital motion. (4) Latitude-dependent variations of H I, He I, and Fe II features reveal aspects of wind behavior during the event. In addition, we discuss implications of the observations for several crucial unsolved problems

CO, Water, and Tentative Methanol in η Carinae Approaching Periastron

The complex circumstellar environment around the massive binary and luminous blue variable η Carinae is known to harbor numerous light molecules, emitting most strongly in rotational states with upper level energies to ~300 K. In circumstellar gas, the complex organic molecule methanol (CH₃OH) has been found almost exclusively around young stellar objects, and thus regarded as a signpost of recent star formation. Here we report the first potential detection of methanol around a highly evolved high-mass star, while using the Atacama Large Millimeter Array to investigate molecular cloud conditions traced by CO (2–1) in an orbit phase preceding the 2020 periastron. The methanol emission originates from hot (T_(gas) ≃ 700 K) material, ~2'' (0.02 pc) across, centered on the dust-obscured binary, and is accompanied by prominent absorption of continuum radiation in a cooler (T_(gas) ≃ 110 K) layer of gas. We also report a first detection of water in Herschel observations at 557 and 988 GHz. The methanol abundance is several to 50 times higher than observed toward several lower-mass stars, while water abundances are similar to those observed in cool, dense molecular clouds. The very high methanol:water abundance ratio in the core of η Car may suggest methanol formation processes similar to Fischer–Tropsch-type catalytic reactions on dust grains. These observations prove that complex molecule formation can occur in a chemically evolved massive stellar environment, given sufficient gas densities and shielding conditions as may occur in material around massive interacting companions and merger remnants

Luminous blue variables: An imaging perspective on their binarity and near environment

Luminous blue variables (LBVs) are rare massive stars with very high luminosity. They are characterized by strong photo-metric and spectroscopic variability related to transient eruptions. The mechanisms at the origin of these eruptions is not well known. In addition, their formation is still problematic and the presence of a companion could help to explain how they form. Aims. This article presents a study of seven LBVs (about 20% of the known Galactic population), some Wolf-Rayet stars, and massive binaries. We probe the environments that surround these massive stars with near-, mid-, and far-infrared images, investigating potential nebula/shells and the companion stars. Methods. To investigate large spatial scales, we used seeing-limited and near diffraction-limited adaptive optics images to obtain a differential diagnostic on the presence of circumstellar matter and to determine their extent. From those images, we also looked for the presence of binary companions on a wide orbit. Once a companion was detected, its gravitational binding to the central star was tested. Tests include the chance projection probability, the proper motion estimates with multi-epoch observations, flux ratio, and star separations. Results. We find that two out of seven of LBVs may have a wide orbit companion. Most of the LBVs display a large circumstellar envelope or several shells. In particular, HD168625, known for its rings, possesses several shells with possibly a large cold shell at the edge of which the rings are formed. For the first time, we have directly imaged the companion of LBV stars