arXiv subjects

Kris Davidson

Publications and source records attributed to Kris Davidson.

At least 37 records · Page 2Linked to original sources

arXiv2013

Luminous and Variable Stars in M31 and M33. I. The Warm Hypergiants and Post-Red Supergiant Evolution

The progenitors of Type IIP supernovae have an apparent upper limit to their initial masses of about 20 solar masses, suggesting that the most massive red supergiants evolve to warmer temperatures before their terminal explosion. But very few post-red supergiants are known. We have identified a small group of luminous stars in M31 and M33 that are candidates for post-red supergiant evolution. These stars have A -- F-type supergiant absorption line spectra and strong hydrogen emission. Their spectra are also distinguished by the Ca II triplet and [Ca II] doublet in emission formed in a low density circumstellar environment. They all have significant near- and mid-infrared excess radiation due to free-free emission and thermal emission from dust. We estimate the amount of mass they have shed and discuss their wind parameters and mass loss rates which range from a few times$ 10^-6 to 10^-4 solar masses/yr. On an HR Diagram, these stars will overlap the region of the LBVs at maximum light, however the warm hypergiants are not LBVs. Their non-spherical winds are not optically thick and they have not exhibited any significant variability. We suggest, however, that the warm hypergiants may be the progenitors of the "less luminous" LBVs such as R71 and even SN1987A.

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arXiv2013

Inverted Ionization Zones in Eta Carinae's "Weigelt Knots"

The Weigelt knots, dense slow-moving ejecta near eta Car, are mysterious in structure as well as in origin. Using a special set of spectrograms obtained with the HST/STIS, we have partially resolved the ionization zones of one knot to an accuracy of about 10 mas. Contrary to simple models, higher ionization levels occur on the_outer_ side of the knot, i.e., farther from the star. They cannot represent a bow shock, and no truly satisfying explanation is yet available -- though we sketch one tentative possibility. We emphasize that 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.

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arXiv2012

The Unusual Temporal and Spectral Evolution of SN2011ht. II. Peculiar Type IIn or Impostor?

SN2011ht has been described both as a true supernova and as an impostor. In this paper, we conclude that it does not match some basic expectations for a core-collapse event. We discuss SN2011ht's spectral evolution from a hot dense wind to a cool dense wind, followed by the post-plateau appearance of a faster low density wind during a rapid decline in luminosity. We identify a slow dense wind expanding at only 500--600 km/s, present throughout the eruption. A faster wind speed V ~ 900 km/s may be identified with a second phase of the outburst. There is no direct or significant evidence for any flow speed above 1000 km/s; the broad asymmetric wings of Balmer emission lines in the hot wind phase were due to Thomson scattering, not bulk motion. We estimate a mass loss rate of order 0.04 Msun/yr during the hot dense wind phase of the event. There is no evidence that the kinetic energy substantially exceeded the luminous energy, roughly 2 X 10^49 ergs; so the total energy was far less than a true SN. We suggest that SN2011ht was a giant eruption driven by super-Eddington radiation pressure, perhaps beginning about 6 months before the discovery. A strongly non-spherical SN might also account for the data, at the cost of more free parameters.

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arXiv2012

The Great Eruption of Eta Carinae

During the years 1838-1858, the very massive star η Carinae became the prototype supernova impostor: it released nearly as much light as a supernova explosion and shed an impressive amount of mass, but survived as a star.1 Based on a light-echo spectrum of that event, Rest et al.2 conclude that "a new physical mechanism" is required to explain it, because the gas outflow appears cooler than theoretical expectations. Here we note that (1) theory predicted a substantially lower temperature than they quoted, and (2) their inferred observational value is quite uncertain. Therefore, analyses so far do not reveal any significant contradiction between the observed spectrum and most previous discussions of the Great Eruption and its physics.

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arXiv2012

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.

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arXiv2011

The Photometric and Spectral Evolution of the 2008 Luminous Optical Transient in NGC 300

The 2008 optical transient in NGC 300 is one of a growing class of intermediate-luminosity transients that brighten several orders of magnitude from a previously optically obscured state. The origin of their eruptions is not understood. Our multi-wavelength photometry and spectroscopy from maximum light to more than a year later provide a record of its post-eruption behavior. We describe its changing spectral-energy distribution, the evolution of its absorption- and emission line-spectrum, the development ofa bipolar outflow, and the rapid transition from a dense wind to an optically thin ionized wind. In addition to strong, narrow hydrogen lines, the F-type absorption-line spectrum of the transient is characterized by strong Ca II and [Ca II] emission. The very broad wings of the Ca II triplet and the asymmetric [Ca II] emission lines are due to strong Thomson scattering in the expanding ejecta. Post-maximum, the hydrogen and Ca II lines developed double-peaked emission profiles that we attribute to a bipolar outflow. Between approximately 60 and 100 days after maximum, the F-type absorption spectrum, formed in its dense wind, weakened and the wind became transparent to ionizing radiation. We discuss the probable evolutionary state of the transient and similar objects such as SN 2008S, and conclude thatthey were most likely post-red supergiants or post-AGB stars on a blue loop to warmer temperatures when the eruption occurred. These objects are not LBVs.

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arXiv2011

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.

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arXiv2011

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.

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arXiv2011

Penetrating the Homunculus -- Near-Infrared Adaptive Optics Images of Eta Carinae

Near-infrared adaptive optics imaging with NICI and NaCO reveal what appears to be a three-winged or lobed pattern, the "butterfly nebula", outlined by bright Br$γ$ and H$_{2}$ emission and light scattered by dust. In contrast, the [Fe II] emission does not follow the outline of the wings, but shows an extended bipolar distribution which is tracing the Little Homunculus ejected in $η$ Car's second or lesser eruption in the 1890's. Proper motions measured from the combined NICI and NaCO images together with radial velocities show that the knots and filaments that define the bright rims of the butterfly were ejected at two different epochs corresponding approximately to the great eruption and the second eruption. Most of the material is spatially distributed 10$\arcdeg$ to 20$\arcdeg$ above and below the equatorial plane apparently behind the Little Homunculus and the larger SE lobe. The equatorial debris either has a wide opening angle or the clumps were ejected at different latitudes relative to the plane. The butterfly is not a coherent physical structure or equatorial torus but spatially separate clumps and filaments ejected at different times, and now 2000 to 4000 AU from the star.

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arXiv2010

Mapping the Asymmetric Thick Disk: III. The Kinematics and Interaction with the Galactic Bar

In the first two papers of this series, Larsen et al (2010a,b) describe our faint CCD survey in the inner Galaxy and map the over-density of Thick Disk stars in Quadrant I (Q1) to 5 kpc or more along the line of sight. The regions showing the strongest excess are above the density contours of the bar in the Galactic disk. In this third paper on the asymmetric Thick Disk, we report on radial velocities and derived metallicity parameters for over 4000 stars in Q1, above and below the plane and in Q4 above the plane. We confirm the corresponding kinematic asymmetry first reported by Parker et al. (2004), extended to greater distances and with more spatial coverage. The Thick Disk stars in Q1 have a rotational lag of 60 -- 70 km/s relative to circular rotation, and the Metal-Weak Thick Disk stars have an even greater lag of 100 km/s. Both lag their corresponding populations in Q4 by approximately 30 km/s. Interestingly, the Disk stars in Q1 also appear to participate in the rotational lag by about 30 km/s. The enhanced rotational lag for the Thick Disk in Q1 extends to 4 kpc or more from the Sun. At 3 to 4 kpc, our sight lines extend above the density contours on the near side of the bar, and as our lines of sight pass directly over the bar the rotational lag appears to decrease. This is consistent with a "gravitational wake" induced by the rotating bar in the Disk which would trap and pile up stars behind it. We conclude that a dynamical interaction with the stellar bar is the most probable explanation for the observed kinematic and spatial asymmetries.

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arXiv2010

The Morphology of IRC +10420's Circumstellar Ejecta

Images of the circumstellar ejecta associated with the post-red supergiant IRC +10420 show a complex ejecta with visual evidence for episodic mass loss. In this paper we describe the transverse motions of numerous knots, arcs and condensations in the inner ejecta measured from second epoch {\it HST/WFPC2} images. When combined with the radial motions for several of the features, the total space motion and direction of the outflows show that they were ejected at different times, in different directions, and presumably from separate regions on the surface of the star. These discrete structures in the ejecta are kinematically distinct from the general expansion of the nebula and their motions are dominated by their transverse velocities. They are apparently all moving within a few degrees of the plane of the sky. We are thus viewing IRC +10420 nearly pole-on and looking nearly directly down onto its equatorial plane. We also discuss the role of surface activity and magnetic fields on IRC +10420's recent mass loss history.

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arXiv2010

A Sea Change in Eta Carinae

Major stellar-wind emission features in the spectrum of Eta Car have recently decreased by factors of order 2 relative to the continuum. This is unprecedented in the modern observational record. The simplest, but unproven, explanation is a rapid decrease in the wind density.

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arXiv2010

Mid-Cycle Changes in Eta Carinae

In late 2006, ground-based photometry of $η$ 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 $η$ 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.

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arXiv2009

High-excitation emission lines near eta Carinae, and its likely companion star

In order to study the distribution of gas and ionizing radiation around eta 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 HST/STIS instrument during 1998-2004. Our principal results, some of them unexpected, are: (1) The high-excitation fluxes varied systematically and non-trivially throughout eta Car's 5.5-year spectroscopic cycle. Instead of rising to a plateau after the 1998 "event," 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 "Weigelt knots" several hundred AU northwest of the star. With less certainty, [Ne III] appears to be somewhat more concentrated than [Fe III]. (4) A faster, blue-shifted component of each feature appears concentrated near the star and elongated perpendicular to the system's 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. Teff = 39000 K and L = 400000 Lsun, for example, would be satisfactory. The allowed region in parameter space is wider (and mostly less luminous) than some previous authors suggested.

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arXiv2008

The Early Spectra of Eta Carinae 1892 to 1941 and the Onset of Its High Excitation Emission Spectrum

The observed behavior of eta Car from 1860 to 1940 has not been considered in most recent accounts, nor has it been explained in any quantitative model. We have used modern digital processing techniques to examine Harvard objective-prism spectra made from 1892 to 1941. Relatively high-excitation He I 4471 and [Fe III] 4658 emission, conspicuous today, were weak and perhaps absent throughout those years. Feast et al. noted this qualitative fact for other pre-1920 spectra, but we quantify it and extend it to a time only three years before Gaviola's first observations of the high-excitation features. Evidently the supply of helium-ionizing photons(lambda < 504A) grew rapidly between 1941 and 1944. The apparent scarcity of such far-UV radiation before 1944 is difficult to explain in models that employ a hot massive secondary star,} because no feasible dense wind or obscuration by dust would have hidden the photoionization caused by the proposed companion during most of its orbital period. We also discuss the qualitative near-constancy of the spectrum from 1900 to 1940, and eta Car's photometric and spectroscopic transition between 1940 and 1953.

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arXiv2007

X-ray Spectral Variation of Eta Carinae through the 2003 X-ray Minimum

We report the results of an X-ray observing campaign on the massive, evolved star Eta Carinae, concentrating on the 2003 X-ray minimum as seen by the XMM-Newton observatory. These are the first spatially-resolved X-ray monitoring observations of the stellar X-ray spectrum during the minimum. The hard X-ray emission, believed to be associated with the collision of Eta Carinae's wind with the wind from a massive companion star, varied strongly in flux on timescales of days, but not significantly on timescales of hours. The lowest X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the maximum seen by RXTE just before the X-ray minimum. The slope of the X-ray continuum above 5 keV did not vary in any observation, which suggests that the electron temperature of the hottest plasma associated with the stellar source did not vary significantly at any phase. Through the minimum, the absorption to the stellar source increased by a factor of 5-10 to NH ~3-4E23 cm-2. The thermal Fe XXV emission line showed significant excesses on both the red and blue sides of the line outside the minimum and exhibited an extreme red excess during the minimum. The Fe fluorescence line at 6.4 keV increased in equivalent width from 100 eV outside the minimum to 200 eV during the minimum. From these observed features, we discuss two possible causes of the X-ray minimum; the eclipse of the X-ray plasma and an intrinsic fading of the X-ray emissivity. The drop in the colliding wind X-ray emission also revealed the presence of an additional X-ray component which exhibited no variation on timescales of weeks to years. This component may be produced by the collision of high speed outflows at v \~1000-2000 km s-1 from Eta Carinae with ambient gas within a few thousand AU from the star.

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arXiv2006

The Chrysalis Opens? Photometry from the Eta Carinae HST Treasury Project, 2002-2006

During the past decade Eta Car has brightened markedly, possibly indicating a change of state. Here we summarize photometry gathered by the Hubble Space Telescope as part of the HST Treasury Project on this object. Our data include STIS/CCD acquisition images, ACS/HRC images in four filters, and synthetic photometry in flux-calibrated STIS spectra. The HST's spatial resolution allows us to examine the central star separate from the bright circumstellar ejecta. Its apparent brightness continued to increase briskly during 2002--06, especially after the mid-2003 spectroscopic event. If this trend continues, the central star will soon become brighter than its ejecta, quite different from the state that existed only a few years ago. One precedent may be the rapid change observed in 1938--1953. We conjecture that the star's mass-loss rate has been decreasing throughout the past century.

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arXiv2006

Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula

We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and High-resolution Spitzer IRS spectra of selected positions within the nebula, and a near-infrared ground-based image made in the light of [Fe II]1.644 micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0 micron, resembles the general morphology of visible H-alpha and near-IR [Fe II] line emission, while the 3.6 and 4.5 micron images are dominated by continuum synchrotron emission. The 24 micron and 70 micron images show enhanced emission that may be due to line emission or the presence of a small amount of warm dust in the nebula on the order of less than 1% of a solar mass. The ratio of the 3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power law index ranging from approximately 0.3 to 0.8 across the nebula. Combining this information with optical and X-ray synchrotron images, we derive a broadband spectrum that reflects the superposition of the flatter spectrum jet and torus with the steeper diffuse nebula, and suggestions of the expected pileup of relativistic electrons just before the exponential cutoff in the X-ray. The pulsar, and the associated equatorial toroid and polar jet structures seen in Chandra and HST images (Hester et al. 2002) can be identified in all of the IRAC images. We present the IR photometry of the pulsar. The forbidden lines identified in the high resolution IR spectra are all double due to Doppler shifts from the front and back of the expanding nebula and give an expansion velocity of approximately 1264 km/s.

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