arXiv subjects

M. Teodoro

Publications and source records attributed to M. Teodoro.

14 recordsLinked to original sources

arXiv2023

The long-term spectral changes of eta Carinae: are they caused by a dissipating occulter as indicated by CMFGEN models?

Eta Carinae ($η$\,Car) exhibits a unique set of P Cygni profiles with both broad and narrow components. Over many decades, the spectrum has changed -- there has been an increase in observed continuum fluxes and a decrease in FeII and HI emission line equivalent widths. The spectrum is evolving towards that of a P Cygni star such as P~Cygni itself and HDE~316285. The spectral evolution has been attributed to intrinsic variations such as a decrease in the mass-loss rate of the primary star or differential evolution in a latitudinal-dependent stellar wind. However intrinsic wind changes conflict with three observational results: the steady long-term bolometric luminosity; the repeating X-ray light curve over the binary period; and the constancy of the dust-scattered spectrum from the Homunculus. We extend previous work that showed a secular strengthening of P~Cygni absorptions by adding more orbital cycles to overcome temporary instabilities and by examining more atomic transitions. {\sc cmfgen} modeling of the primary wind shows that a time-decreasing mass-loss rate is not the best explanation for the observations. However, models with a `small' dissipating absorber in our line-of-site can explain both the increase in brightness and changes in the emission and P Cygni absorption profiles. If the spectral evolution is caused by the dissipating circumstellar medium, and not by intrinsic changes in the binary, the dynamical timescale to recover from the Great Eruption is much less than a century, different from previous suggestions.

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arXiv2020

On the changes in the physical properties of the ionized region around the Weigelt structures in Eta Carinae over the 5.54-yr spectroscopic cycle

We present HST/STIS observations and analysis of two prominent nebular structures around the central source of Eta 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) that the total extinction towards the Weigelt structures is that the total extinction towards the Weigelt structures is AsubV =2/0. that the total extinction towards the Weigelt structures is AV = 2.0. Weigelt C and D are characterized by an electron density of that the total extinction towards the Weigelt structures is AV = 2.0. Weigelt C and D are characterized by an electron density of 10exp6.9 cm-3 that does not significantly change throughout the orbital cycle. The electron temperature varies from 5500 K (around periastron) to 7200 K (around apastron). The relative changes in the brightness of 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 the Butterfly Nebula surrounding the central binary system.

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arXiv2016

VLTI-AMBER velocity-resolved aperture-synthesis imaging of Eta Carinae with a spectral resolution of 12000. Studies of the primary star wind and innermost wind-wind collision

Context. The mass loss from massive stars is not understood well. Eta Car is a unique object for studying the massive stellar wind during the LBV phase. It is also an eccentric binary with a period of 5.54 yr. The nature of both stars is uncertain, although we know from X-ray studies that there is a wind-wind collision whose properties change with orbital phase. Methods. Observations of Eta Car were carried out with the ESO VLTI and the AMBER instrument between approximately five and seven months before the August 2014 periastron passage. Velocity-resolved aperture-synthesis images were reconstructed from the spectrally dispersed interferograms. Interferometric studies can provide information on the binary orbit, the primary wind, and the wind collision. Results. We present velocity-resolved aperture-synthesis images reconstructed in more than 100 different spectral channels distributed across the Br Gamma 2.166 micrometer emission line. The intensity distribution of the images strongly depends on wavelength. At wavelengths corresponding to radial velocities of approximately -140 to -376 km/s measured relative to line center, the intensity distribution has a fan-shaped structure. At the velocity of -277 km/s, the position angle of the symmetry axis of the fan is ~ 126 degree. The fan-shaped structure extends approximately 8.0 mas (~ 18.8 au) to the southeast and 5.8 mas (~ 13.6 au) to the northwest, measured along the symmetry axis at the 16% intensity contour. The shape of the intensity distributions suggests that the obtained images are the first direct images of the innermost wind-wind collision zone. Therefore, the observations provide velocity-dependent image structures that can be used to test three-dimensional hydrodynamical, radiative transfer models of the massive interacting winds of Eta Car.

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arXiv2016

He II $λ$4686 emission from the massive binary system in $η$ Car: constraints to the orbital elements and the nature of the periodic minima

η Carinae is an extremely massive binary system in which rapid spectrum variations occur near periastron. Most notably, near periastron the He II $λ4686$ line increases rapidly in strength, drops to a minimum value, then increases briefly before fading away. To understand this behavior, we conducted an intense spectroscopic monitoring of the He II $λ4686$ emission line across the 2014.6 periastron passage using ground- and space-based telescopes. Comparison with previous data confirmed the overall repeatability of EW(He II $λ4686$), the line radial velocities, and the timing of the minimum, though the strongest peak was systematically larger in 2014 than in 2009 by 26%. The EW(He II $λ4686$) variations, combined with other measurements, yield an orbital period $2022.7\pm0.3$ d. The observed variability of the EW(He II $λ4686$) was reproduced by a model in which the line flux primarily arises at the apex of the wind-wind collision and scales inversely with the square of the stellar separation, if we account for the excess emission as the companion star plunges into the hot inner layers of the primary's atmosphere, and including absorption from the disturbed primary wind between the source and the observer. This model constrains the orbital inclination to $135^\circ$-$153^\circ$, and the longitude of periastron to $234^\circ$-$252^\circ$. It also suggests that periastron passage occurred on $T_0 = 2456874.4\pm1.3$ d. Our model also reproduced EW(He II $λ4686$) variations from a polar view of the primary star as determined from the observed He II $λ4686$ emission scattered off the Homunculus nebula.

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arXiv2015

The X-ray Lightcurve of the Supermassive star eta Carinae, 1996--2014

Eta Carinae is the nearest example of a supermassive, superluminous, unstable star. Mass loss from the system is critical in shaping its circumstellar medium and in determining its ultimate fate. Eta Car currently loses mass via a dense, slow stellar wind and possesses one of the largest mass loss rates known. It is prone to episodes of extreme mass ejection via eruptions from some as-yet unspecified cause; the best examples of this are the large-scale eruptions which occurred in 19th century. Eta Car is a colliding wind binary in which strong variations in X-ray emission and in other wavebands are driven by the violent collision of the wind of eta Car-A and the fast, less dense wind of an otherwise hidden companion star. X-ray variations are the simplest diagnostic we have to study the wind-wind collision and allow us to measure the state of the stellar mass loss from both stars. We present the X-ray lightcurve over the last 20 years from ROSAT observations and monitoring with the Rossi X-ray Timing Explorer and the X-ray Telescope on the Swift satellite. We compare and contrast the behavior of the X-ray emission from the system over that timespan, including surprising variations during the 2014 X-ray minimum.

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arXiv2014

The three-dimensional structure of the Eta Carinae Homunculus

We investigate, using the modeling code SHAPE, the three-dimensional structure of the bipolar Homunculus nebula surrounding Eta Carinae, as mapped by new ESO VLT/X-Shooter observations of the H2 $λ=2.12125$ micron emission line. Our results reveal for the first time important deviations from the axisymmetric bipolar morphology: 1) circumpolar trenches in each lobe positioned point-symmetrically from the center and 2) off-planar protrusions in the equatorial region from each lobe at longitudinal (~55 degrees) and latitudinal (10-20 degrees) distances from the projected apastron direction of the binary orbit. The angular distance between the protrusions (~110 degrees) is similar to the angular extent of each polar trench (~130 degrees) and nearly equal to the opening angle of the wind-wind collision cavity (~110 degrees). As in previous studies, we confirm a hole near the centre of each polar lobe and no detectable near-IR H2 emission from the thin optical skirt seen prominently in visible imagery. We conclude that the interaction between the outflows and/or radiation from the central binary stars and their orientation in space has had, and possibly still has, a strong influence on the Homunculus. This implies that prevailing theoretical models of the Homunculus are incomplete as most assume a single star origin that produces an axisymmetric nebula. We discuss how the newly found features might be related to the Homunculus ejection, the central binary and the interacting stellar winds. We also include a 3D printable version of our Homunculus model.

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arXiv2013

Detection of the compressed primary stellar wind in eta Carinae

A series of three HST/STIS spectroscopic mappings, spaced approximately one year apart, reveal three partial arcs in [Fe II] and [Ni II] emissions moving outward from eta Carinae. We identify these arcs with the shell-like structures, seen in the 3D hydrodynamical simulations, formed by compression of the primary wind by the secondary wind during periastron passages.

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arXiv2011

He II 4686 in eta Carinae: collapse of the wind-wind collision region during periastron passage

The periodic spectroscopic events in eta Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit. Several mechanisms have been proposed to explain the variations of different spectral features, such as an eclipse by the wind-wind collision boundary, a shell ejection from the primary star or accretion of its wind onto the secondary. All of them have problems explaining all the observed phenomena. To better understand the nature of the cyclic events, we performed a dense monitoring of eta Carinae with 5 Southern telescopes during the 2009 low excitation event, resulting in a set of data of unprecedented quality and sampling. The intrinsic luminosity of the He II 4686 emission line (L~310 Lsun) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the wind-wind collision (WWC) region. Clumps in the primary's wind probably explain the flare-like behavior of both the X-ray and He II 4686 light-curves. After a short-lived minimum, He II 4686 emission rises again to a ne

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arXiv2010

Eta Carinae long-term variability

We present preliminary results of our analysis on the long-term variations observed in the optical spectrum of the LBV star Eta Carinae. Based on the hydrogen line profiles, we conclude that the physical parameters of the primary star did not change in the last 15 years.

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arXiv2008

Near-infrared integral field spectroscopy of the Homunculus nebula around Eta Carinae using Gemini/CIRPASS

This work presents the first integral field spectroscopy of the Homunculus nebula around Eta Carinae in the near-infrared spectral region (J band). We confirmed the presence of a hole on the polar region of each lobe, as indicated by previous near-IR long-slit spectra and mid-IR images. The holes can be described as a cylinder of height (i.e. the thickness of the lobe) and diameter of 6.5 and 6.0x10^{16} cm, respectively. We also mapped the blue-shifted component of He I 10830 seen towards the NW lobe. Contrary to previous works, we suggested that this blue-shifted component is not related to the Paddle but it is indeed in the equatorial disc. We confirmed the claim of Smith (2005) and showed that the spatial extent of the Little Homunculus matches remarkably well the radio continuum emission at 3 cm, indicating that the Little Homunculus can be regarded as a small HII region. Therefore, we used the optically-thin 1.3 mm radio flux to derive a lower limit for the number of Lyman-continuum photons of the central source in Eta Car. In the context of a binary system, and assuming that the ionising flux comes entirely from the hot companion star, the lower limit for its spectral type and luminosity class ranges from O5.5 III to O7 I. Moreover, we showed that the radio peak at 1.7 arcsec NW from the central star is in the same line-of-sight of the `Sr-filament' but they are obviously spatially separated, while the blue-shifted component of He I 10830 may be related to the radio peak and can be explained by the ultraviolet radiation from the companion star.

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arXiv2008

A multispectral view of the periodic events in eta Carinae

A full description of the 5.5-yr low excitation events in Eta Carinae is presented. We show that they are not as simple and brief as previously thought, but a combination of two components. The first, the 'slow variation' component, is revealed by slow changes in the ionization level of circumstellar matter across the whole cycle and is caused by gradual changes in the wind-wind collision shock-cone orientation, angular opening and gaseous content. The second, the 'collapse' component, is restricted to around the minimum, and is due to a temporary global collapse of the wind-wind collision shock. High energy photons (E > 16 eV) from the companion star are strongly shielded, leaving the Weigelt objects at low ionization state for >6 months. High energy phenomena are sensitive only to the 'collapse', low energy only to the 'slow variation' and intermediate energies to both components. Simple eclipses and mechanisms effective only near periastron (e.g., shell ejection or accretion onto the secondary star) cannot account for the whole 5.5-yr cycle. We find anti-correlated changes in the intensity and the radial velocity of P Cygni absorption profiles in FeII 6455 and HeI 7065 lines, indicating that the former is associated to the primary and the latter to the secondary star. We present a set of light curves representative of the whole spectrum, useful for monitoring the next event (2009 January 11).

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arXiv2007

The periodicity of the eta Carinae events

Extensive spectral observations of eta Carinae over the last cycle, and particularly around the 2003.5 low excitation event, have been obtained. The variability of both narrow and broad lines, when combined with data taken from two earlier cycles, reveal a common and well defined period. We have combined the cycle lengths derived from the many lines in the optical spectrum with those from broad-band X-rays, optical and near-infrared observations, and obtained a period length of 2022.7+-1.3 d. Spectroscopic data collected during the last 60 years yield an average period of 2020+-4 d, consistent with the present day period. The period cannot have changed by more than $Δ$P/P=0.0007 since 1948. This confirms the previous claims of a true, stable periodicity, and gives strong support to the binary scenario. We have used the disappearance of the narrow component of HeI 6678 to define the epoch of the Cycle 11 minimum, T_0=JD 2,452,819.8. The next event is predicted to occur on 2009 January 11 (+-2 days). The dates for the start of the minimum in other spectral features and broad-bands is very close to this date, and have well determined time delays from the HeI epoch.

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arXiv2006

Detection of additional Wolf-Rayet stars in the starburst cluster Westerlund 1 with SOAR

We report the detection of 3 additional Wolf-Rayet stars in the young cluster Westerlund 1. They were selected as emission-line star candidates based on 1 micron narrow-band imaging of the cluster carried out at OPD/LNA (Brazil), and then confirmed as Wolf-Rayet stars by K-band spectroscopy performed at the 4.1 m SOAR telescope (Chile). Together with previous works, this increases the population of Wolf-Rayet stars detected in the cluster to 22 members. Moreover, it is presented for the first time a K-band spectrum of the luminous blue variable W243, which apparently implies in a higher temperature than that derived from optical spectra taken in 2003. The WC9 star WR-F was also observed, showing clear evidence of dust emission in the K-band.

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arXiv2006

A Rich Population of X-ray Emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1

Recent optical and IR studies have revealed that the heavily-reddened starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (WR) stars, comprising the richest WR population of any galactic cluster. We present results of a senstive Chandra X-ray observation of Wd 1 which detected 12 of the 22 known WR stars and the mysterious emission line star W9. The fraction of detected WN stars is nearly identical to that of WC stars. The WN stars WR-A and WR-B as well as W9 are exceptionally luminous in X-rays and have similar hard heavily-absorbed spectra with strong Si XIII and S XV emission lines. The luminous high-temperature X-ray emission of these three stars is characteristic of colliding wind binary systems but their binary status remains to be determined. Spectral fits of the X-ray bright sources WR-A and W9 with isothermal plane-parallel shock models require high absorption column densities log N$_{H}$ = 22.56 (cm$^{-2}$) and yield characteristic shock temperatures kT_shock ~ 3 keV (T ~ 35 MK).

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