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

High-Excitation Emission Lines Near Eta Carinae, and Its Likely Companion Star

In order to study the distribution of gas and ionizing radiation around η Car and their implications for its likely companion star, we have examined high-excitation emission lines of [Ne III], [Fe III], etc., in spectra obtained with the Hubble Space Telescope/Space Telescope Imaging Spectrograph instrument during 1998-2004. Our principal results, some of them unexpected, are as follows. (1) The high-excitation fluxes varied systematically and non-trivially throughout η Car\u27s 5.5 year spectroscopic cycle. Instead of rising to a plateau after the 1998 \u27\u27event,\u27\u27 they changed continuously with a maximum in mid-cycle. (2) At one significant location a brief, strong secondary maximum occurred just before the 2003.5 spectroscopic event. (3) These emission lines are strongly concentrated at the \u27\u27Weigelt knots\u27\u27 several hundred AU northwest of the star. With less certainty, [Ne III] appears to be somewhat more concentrated than [Fe III]. (4) A faster, blueshifted component of each feature appears concentrated near the star and elongated perpendicular to the system\u27s bipolar axis. This structure may be related to the equatorial outflow and/or to dense material known to exist along our line of sight to the star. (5) Using the photoionization program Cloudy, we estimated the range of parameters for the hot secondary star that would give satisfactory high-excitation line ratios in the ejecta. T eff ≈ 39, 000 K and L ~ 4 × 105 L ☉, for example, would be satisfactory. The allowed region in parameter space is wider (and mostly less luminous) than some previous authors suggested. This research was supported by grant nos. GO-9973, 10844, and 11291 from the Space Telescope Science Institute. The HST is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555

Secular Changes in Eta Carinae's Wind 1998-2011

Stellar wind-emission features in the spectrum of eta Carinae have decreased by factors of 1.5-3 relative to the continuum within the last 10 years. We investigate a large data set from several instruments (STIS, GMOS, UVES) obtained between 1998 and 2011 and we analyze the progression of spectral changes in the direct view of the star, in the reflected polar-on spectra at FOS4, and at the Weigelt knots. We find that the spectral changes occurred gradually on a time scale of about 10 years and that they are dependent on the viewing angle. The line strengths declined most in our direct view of the star. About a decade ago, broad stellar wind-emission features were much stronger in our line-of-sight view of the star than at FOS4. After the 2009 event, the wind-emission line strengths are now very similar at both locations. High-excitation He I and N II absorption lines in direct view of the star strengthened gradually. The terminal velocity of Balmer P Cyg absorption lines now appears to be less latitude-dependent and the absorption strength may have weakened at FOS4. Latitude-dependent alterations in the mass-loss rate and the ionization structure of eta Carinae's wind are likely explanations for the observed spectral changes.Comment: 42 pages, 12 figures, 2 table

The LBV HR Car has a partner: Discovery of a companion with the VLTI

Luminous Blue Variables (LBVs) are massive stars caught in a post-main sequence phase, during which they are losing a significant amount of mass. As, on one hand, it is thought that the majority of massive stars are close binaries that will interact during their lifetime, and on the other, the most dramatic example of an LBV, Eta Car, is a binary, it would be useful to find other binary LBVs. We present here interferometric observations of the LBV HR Car done with the AMBER and PIONIER instruments attached to ESO's Very Large Telescope Interferometer (VLTI). Our observations, spanning two years, clearly reveal that HR Car is a binary star. It is not yet possible to constrain fully the orbit, and the orbital period may lie between a few years and several hundred years. We derive a radius for the primary in the system and possibly resolve as well the companion. The luminosity ratio in the H-band between the two components is changing with time, going from about 6 to 9. We also tentatively detect the presence of some background flux which remained at the 2% level until January 2016, but then increased to 6% in April 2016. Our AMBER results show that the emission line forming region of Br gamma is more extended than the continuum emitting region as seen by PIONIER and may indicate some wind-wind interaction. Most importantly, we constrain the total masses of both components, with the most likely range being 33.6 and 45 solar masses. Our results show that the LBV HR Car is possibly an Eta Car analog binary system with smaller masses, with variable components, and further monitoring of this object is definitively called for.Comment: A&A, in pres

The possible disappearance of a massive star in the low metallicity galaxy PHL 293B

We investigate a suspected very massive star in one of the most metal-poor dwarf galaxies, PHL~293B. Excitingly, we find the sudden disappearance of the stellar signatures from our 2019 spectra, in particular the broad H lines with P~Cygni profiles that have been associated with a massive luminous blue variable (LBV) star. Such features are absent from our spectra obtained in 2019 with the ESPRESSO and X-shooter instruments of the ESO's VLT. We compute radiative transfer models using CMFGEN that fit the observed spectrum of the LBV and are consistent with ground-based and archival Hubble Space Telescope photometry. Our models show that during 2001--2011 the LBV had a luminosity $L_* = 2.5-3.5 \times 10^6 ~L_{\odot}$, a mass-loss rate $\dot{M} = 0.005-0.020 ~M_{\odot}$~yr$^{-1}$, a wind velocity of 1000~km~s$^{-1}$, and effective and stellar temperatures of $T_\mathrm{eff} = 6000-6800$~K and $T_\mathrm{*}=9500-15000$~K. These stellar properties indicate an eruptive state. We consider two main hypotheses for the absence of the broad emission components from the spectra obtained since 2011. One possibility is that we are seeing the end of an LBV eruption of a surviving star, with a mild drop in luminosity, a shift to hotter effective temperatures, and some dust obscuration. Alternatively, the LBV could have collapsed to a massive black hole without the production of a bright supernova.Comment: 8, pages, 7 figures, MNRAS accepted ; see also the ESO press release at: https://www.eso.org/public/news/eso2010