Abstract
We report high-precision X-ray monitoring observations in the 0.4–10 keV band of the luminous, long-period colliding wind binary Eta Carinae, up to and through its most recent X-ray ...minimum/periastron passage in 2020 February. Eta Carinae reached its observed maximum X-ray flux on 2020 January 7, at a flux level of 3.30 ×10
−10
ergs s
−1
cm
−2
, followed by a rapid plunge to its observed minimum flux, 0.03 × 10
−10
ergs s
−1
cm
−2
, near 2020 February 17. The NICER observations show an X-ray recovery from the minimum of only ∼16 days, the shortest X-ray minimum observed so far. We provide new constraints for the “deep” and “shallow” minimum intervals. Variations in the characteristic X-ray temperatures of the hottest observed X-ray emission indicate that the apex of the wind–wind “bow shock” enters the companion’s wind acceleration zone about 81 days before the start of the X-ray minimum. There is a steplike increase in column density just before the X-ray minimum, probably associated with the presence of dense clumps near the shock apex. During the recovery and after, the column density shows a smooth decline, which agrees with previous
N
H
measurements made by Swift at the same orbital phase, indicating that the changes in the mass-loss rate are only a few percent over the two cycles. Finally, we use the variations in the X-ray flux of the outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta of ∼10
41
ergs s
−1
near the time of the “Great Eruption.”
Using the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS), observations of the O III emission from the narrow-line region (NLR) of NGC 4151 were obtained and radial ...velocities determined. Five orbits of HST time were used to obtain spectra at five parallel slit configurations at a position angle of 58°, with spatial resolution 02 across and 01 along each slit. A spectral resolving power ( lambda /l) of ~9000 with the G430M grating gave velocity measurements accurate to ~34 km s-1. A kinematic model was generated to match the radial velocities for comparison to previous kinematic models of biconical radial outflow developed for low-dispersion spectra at two slit positions. The new high-resolution spectra permit the measurement of accurate velocity dispersions for each radial velocity component. The FWHM reaches a maximum of 1000 km s-1 near the nucleus and generally decreases with increasing distance to about 100 km s-1 in the extended NLR, starting at about 6'' from the nucleus. In addition to the bright emission knots, which generally fit our model, there are faint high-velocity clouds that do not fit the biconical outflow pattern of our kinematic model. These faint clouds occur at the turnover points of the outflowing bright clouds. We suggest possible scenarios that could explain these rogue clouds: (1) backflow resulting from shocks and (2) outflow outside of the bicones, although the latter does not explain how the knots are ionized and accelerated. A comparison of our observations with a high-resolution radio map shows that there is no evidence that the kinematics of the NLR clouds are affected by the radio lobes that comprise the inner jet.
Eta Carinae is a massive interacting binary system shrouded in a complex circumstellar environment whose evolution is the source of the long-term brightening observed during the last 80 years. An ...occulter, acting as a natural coronagraph, impacts observations from our perspective, but not from most other directions. Other sight-lines are visible to us through studies of the Homunculus reflection nebula. The coronagraph appears to be vanishing, decreasing the extinction towards the central star, and causing the star's secular brightening. In contrast, the Homunculus remains at an almost constant brightness. The coronagraph primarily suppresses the stellar continuum, to a lesser extent the wind lines, and not the circumstellar emission lines. This explains why the absolute values of equivalent widths (EWs) of the emission lines in our direct view are larger than those seen in reflected by the Homunculus, why the direct view absolute EWs are decreasing with time, and why lower-excitation spectral wind lines formed at larger radii (e.g Fe ii 4585A) decrease in intensity at a faster pace than higher excitation lines that form closer to the star (e.g. Hdelta). Our main result is that the star, despite its 10-fold brightening over two decades, is relatively stable. A vanishing coronagraph that can explain both the large flux evolution and the much weaker spectral evolution. This is contrary to suggestions that the long-term variability is intrinsic to the primary star that is still recovering from the Great Eruption with a decreasing mass-loss rate and a polar wind that is evolving at a slower pace than at the equator.
η Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-IR source in the sky outside our solar system. Since the late 1990s, the central source has dramatically ...brightened at UV and optical wavelengths. This might be explained by a decrease in circumstellar dust extinction. We aim to establish the mid-IR flux evolution and further our understanding of the star’s UV and optical brightening. Mid-IR images from 8−20 μm were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 were retrieved from the ESO Science Archive Facility, and historical records were collected from publications. We present mid-IR images of η Car with the highest angular resolution to date at the corresponding wavelengths (≥0.22″). We reconstruct the mid-IR evolution of the spectral energy distribution of the spatially integrated Homunculus nebula from 1968 to 2018 and find no long-term changes. The bolometric luminosity of η Car has been stable over the past five decades. We do not observe a long-term decrease in the mid-IR flux densities that could be associated with the brightening at UV and optical wavelengths, but circumstellar dust must be declining in our line of sight alone. Short-term flux variations within about 25% of the mean levels could be present.
ABSTRACT
The evolved massive binary star η Carinae underwent eruptive mass-loss events that formed the complex bi-polar ‘Homunculus’ nebula harbouring tens of solar masses of unusually nitrogen-rich ...gas and dust. Despite expectations for the presence of a significant molecular component to the gas, detections have been observationally challenged by limited access to the far-infrared and the intense thermal continuum. A spectral survey of the atomic and rotational molecular transitions was carried out with the Herschel Space Observatory, revealing a rich spectrum of broad emission lines originating in the ejecta. Velocity profiles of selected PACS lines correlate well with known substructures: H i in the central core; NH and weak C ii within the Homunculus; and N ii emissions in fast-moving structures external to the Homunculus. We have identified transitions from O i, H i, and 18 separate light C- and O-bearing molecules including CO, CH, CH+, and OH, and a wide set of N-bearing molecules: NH, NH+, N2H+, NH2, NH3, HCN, HNC, CN, and N2H+. Half of these are new detections unprecedented for any early-type massive star environment. A very low ratio 12C/13C ≤ 4 is estimated from five molecules and their isotopologues. We demonstrate that non-LTE effects due to the strong continuum are significant. Abundance patterns are consistent with line formation in regions of carbon and oxygen depletions with nitrogen enhancements, reflecting an evolved state of the erupting star with efficient transport of CNO-processed material to the outer layers. The results offer many opportunities for further observational and theoretical investigations of the molecular chemistry under extreme physical and chemical conditions around massive stars in their final stages of evolution.
ABSTRACT
We present HST/STIS observations and analysis of two prominent nebular structures around the central source of η Carinae, the knots C and D. The former is brighter than the latter for ...emission lines from intermediate- or high-ionization potential ions. The brightness of lines from intermediate- and high-ionization potential ions significantly decreases at phases around periastron. We do not see conspicuous changes in the brightness of lines from low-ionization potential (<13.6 eV) ions over the orbital period. Line ratios suggest that the total extinction towards the Weigelt structures is AV = 2.0. Weigelt C and D are characterized by an electron density of 106.9 cm−3 that does not significantly change throughout the orbital cycle. The electron temperature varies from 5500 (around periastron) to 7200 K (around apastron). The relative changes in the brightness of the He i lines are well reproduced by the variations in the electron temperature alone. We found that, at phases around periastron, the electron temperature seems to be higher for Weigelt C than that of D. The Weigelt structures are located close to the Homunculus equatorial plane, at a distance of about 1240 au from the central source. From the analysis of proper motion and age, the Weigelt complex can be associated with the equatorial structure called ‘Butterfly Nebula’ surrounding the central binary system.
Abstract
We report on
Swift
X-ray Telescope observations of Eta Carinae (
η
Car), an extremely massive, long-period, highly eccentric binary obtained during the 2014.6 X-ray minimum/periastron ...passage. These observations show that
η
Car may have been particularly bright in X-rays going into the X-ray minimum state, while the duration of the 2014 X-ray minimum was intermediate between the extended minima seen in 1998.0 and 2003.5 by
Rossi X-Ray Timing Explorer
(
RXTE
), and the shorter minimum in 2009.0. The hardness ratios derived from the
Swift
observations showed a relatively smooth increase to a peak value occurring 40.5 days after the start of the X-ray minimum, though these observations cannot reliably measure the X-ray hardness during the deepest part of the X-ray minimum when contamination by the “central constant emission” component is significant. By comparing the timings of the
RXTE
and
Swift
observations near the X-ray minima, we derive an updated X-ray period of
P
X
= 2023.7 ± 0.7 days, in good agreement with periods derived from observations at other wavelengths, and we compare the X-ray changes with variations in the He
ii
4686 emission. The middle of the “Deep Minimum” interval, as defined by the
Swift
column density variations, is in good agreement with the time of periastron passage derived from the He
ii
λ
4686 line variations.
The highly eccentric binary system Eta Carinae (... Car) shows numerous time-variable emission and absorption features. These observational signatures are the result of interactions between the ...complex three-dimensional (3D) wind-wind collision regions and photoionization by the luminous stars. Specifically, helium presents several interesting spectral features that provide important clues on the geometry and physical proprieties of the system and the individual stars. We use the simplex algorithm to post-process 3D smoothed particle hydrodynamics simulation output of the interacting winds in ... Car in order to obtain the fractions of ionized helium assuming three different primary star (...) mass-loss rates. The resultant ionization maps constrain the regions where helium is singly- and doubly-ionized. We find that reducing ...A's mass-loss rate (M...) increases the volume of He+. Lowering M... produces large variations in the volume of He+ in the pre-shock ... wind on the periastron side of the system. Our results show that binary orientations in which apastron is on our side of the system are more consistent with available observations. We suggest that small variations in M... might explain the observed increase in He i absorption in recent decades, although numerous questions regarding this scenario remain open. We also propose that the absence of broad He...i lines in the spectra of ... Car between its 1890's eruption and ~1944 might be explained by ...'s He...-ionizing photons not being able to penetrate the wind-wind interaction region, due to a higher M... at that time (by a factor ...2, compared to the present value). (ProQuest: ... denotes formulae/symbols omitted.)
We present a three-dimensional (3D) dynamical model for the broad Fe iii emission observed in η Carinae using the Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS). This model is ...based on full 3D smoothed particle hydrodynamics simulations of η Car's binary colliding winds. Radiative transfer codes are used to generate synthetic spectroimages of Fe iii emission-line structures at various observed orbital phases and STIS slit position angles (PAs). Through a parameter study that varies the orbital inclination i, the PA θ that the orbital plane projection of the line of sight makes with the apastron side of the semimajor axis and the PA on the sky of the orbital axis, we are able, for the first time, to tightly constrain the absolute 3D orientation of the binary orbit. To simultaneously reproduce the blueshifted emission arcs observed at orbital phase 0.976, STIS slit PA =+38° and the temporal variations in emission seen at negative slit PAs, the binary needs to have an i≈ 130° to 145°, θ≈−15° to +30° and an orbital axis projected on the sky at a PA ≈ 302° to 327° east of north. This represents a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula, in 3D. The companion star, ηB, thus orbits clockwise on the sky and is on the observer's side of the system at apastron. This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modelling to determine the stellar masses.
When massive stars exhaust their fuel, they collapse and often produce the extraordinarily bright explosions known as core-collapse supernovae. On occasion, this stellar collapse also powers an even ...more brilliant relativistic explosion known as a long-duration gamma-ray burst. One would then expect that these long gamma-ray bursts and core-collapse supernovae should be found in similar galactic environments. Here we show that this expectation is wrong. We find that the gamma-ray bursts are far more concentrated in the very brightest regions of their host galaxies than are the core-collapse supernovae. Furthermore, the host galaxies of the long gamma-ray bursts are significantly fainter and more irregular than the hosts of the core-collapse supernovae. Together these results suggest that long-duration gamma-ray bursts are associated with the most extremely massive stars and may be restricted to galaxies of limited chemical evolution. Our results directly imply that long gamma-ray bursts are relatively rare in galaxies such as our own Milky Way.
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