Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the ...mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV’s) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star’s photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature.
ON THE SOCIAL TRAITS OF LUMINOUS BLUE VARIABLES Humphreys, Roberta M.; Weis, Kerstin; Davidson, Kris ...
Astrophysical journal/The Astrophysical journal,
07/2016, Letnik:
825, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT In a recent paper, Smith & Tombleson state that the luminous blue variables (LBVs) in the Milky Way and the Magellanic Clouds are isolated; they are not spatially associated with young ...O-type stars. They propose a novel explanation that would overturn the standard view of LBVs. In this paper we test their hypothesis for the LBVs in M31 and M33, as well as the LMC and SMC. We show that in M31 and M33 the LBVs are associated with luminous young stars and supergiants that are appropriate to their luminosities and positions on the H-R diagram. Moreover, in the Smith and Tombleson scenario most of the LBVs should be runaway stars, but the stars' velocities are consistent with their positions in the respective galaxies. In the Magellanic Clouds, those authors' sample was a mixed population. We reassess their analysis, removing seven stars that have no clear relation to LBVs. When we separate the more massive classical and the less luminous LBVs, the classical LBVs have a distribution similar to the late O-type stars, while the less luminous LBVs have a distribution like the red supergiants. None of the confirmed LBVs have high velocities or are candidate runaway stars. These results support the accepted description of LBVs as evolved massive stars that have shed a lot of mass and are now close to their Eddington limit.
Abstract
Existing
Hubble Space Telescope
UV data offer many previously neglected clues to
η
Car’s behavior since 2000. Here we examine a subset of observations with diverse results. (1) The star’s ...rapid change of state is confirmed by major changes in UV absorption lines, circumstellar extinction, and other features. (2) N
iii
λ
1750 is one of the two most luminous emission features in
η
Car’s observable spectrum, comparable to H
α
. This and other semi-forbidden lines are useful because they have no P Cyg absorption. (3) N
iii
multiplet ratios provide the first direct diagnostic of gas densities in
η
Car’s outflow. They strongly suggest that high-excitation lines originate in condensations within the colliding-wind shocked region. The parameters imply that published models have not adequately represented the essential small size scales. (4) In 2002–2004, a very large amount of N
iii
emission had anomalous Doppler velocities from +400 to +1200 km s
−1
. This is a mystery; we conjecture that it may have resulted from a burst of mass ejection in the 2003.5 periastron event. Various other effects are also difficult to explain and merit further investigation.
Abstract The red hypergiant VY CMa is famous for its very visible record of high-mass-loss events. Recent CO observations with the Atacama Large Millimeter/submillimeter Array (ALMA) revealed three ...previously unknown large-scale outflows (Singh et al). In this paper, we use the CO maps to investigate the motions of a cluster of four clumps close to the star, not visible in the optical or infrared images. We present their proper motions measured from two epochs of ALMA images and determine the line-of-sight velocities of the gas in emission at the clumps. We estimate their masses and ages, or time since ejection, and conclude that all four were ejected during VY CMa’s active period in the early 20th century. Together with two additional knots observed with the Hubble Space Telescope, VY CMa experienced at least six massive outflows during a 30 yr period, with a total mass lost ≥0.07 M ⊙ . The position–velocity map of the 12 CO emission reveals previously unnoticed attributes of the older outer ejecta. In a very narrow range of Doppler velocities, 12 CO absorption and emission causes some of this outer material to be quite opaque. At those frequencies the inner structure is hidden and we see only emission from an extended outer region. This fact produces a conspicuous but illusory dark spot if one attempts to subtract the continuum in a normal way.
SN2011ht has been described both as a true supernova (SN) 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 super(-1), present throughout the eruption. A faster wind speed V ~ 900 km s super(-1) occurred in a second phase of the outburst. There is no direct or significant evidence for any flow speed above 1000 km s super(-1); 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 0.05 M sub(middot in circle) yr super(-1) during the hot dense wind phase of the event. The same calculations present difficulties for a hypothetical unseen SN blast wave. There is no evidence that the kinetic energy greatly exceeded the luminous energy, roughly 3 x 10 super(49) erg; so the radiative plus kinetic energy was small compared to a typical SN. We suggest that SN2011ht may have been a giant eruption driven by super-Eddington radiation pressure, perhaps beginning a few months before the discovery. A strongly non-spherical SN might also account for the data at the cost of more free parameters.
Eta Carinae's spectroscopic events (periastron passages) in 2003, 2009, and 2014 differed progressively. He II lambda4687 and nearby N II multiplet 5 have special significance because they respond to ...very soft X-rays and the ionizing UV radiation field (EUV). Hubble Space Telescope (HST)/STIS observations in 2014 show dramatic increases in both features compared to the previous 2009.1 event. These results appear very consistent with a progressive decline in the primary wind density, proposed years ago on other grounds. If material falls onto the companion star near periastron, the accretion rate may now have become too low to suppress the EUV.
The Weigelt knots, dense slow-moving ejecta near {eta} 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.
SECULAR CHANGES IN ETA CARINAE'S WIND 1998―2011 MEHNER, Andrea; DAVIDSON, Kris; HUMPHREYS, Roberta M ...
Astrophysical journal/The Astrophysical journal,
05/2012, Letnik:
751, Številka:
1
Journal Article
Recenzirano
Odprti dostop
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 analyze the progression of spectral changes in 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 timescale 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.
ETA CARINAE AND ITS ENVIRONMENT Davidson, Kris; Humphreys, Roberta M
Annual review of astronomy and astrophysics,
09/1997, Letnik:
35, Številka:
1
Journal Article
Recenzirano
Eta Carinae (Eta) is one of the most remarkable of all well-studied stars
and perhaps the most poorly understood. Observations with the Hubble Space
Telescope and other modern instruments have solved ...a few of the mysteries
concerning this object while opening a comparable number of new ones. In this
review we first recount some essential background information concerning Eta,
then we sketch most of the observational developments of the past few years,
related to the star itself and to its ejecta. Throughout, we propose a series
of specific unsolved observational and theoretical problems that seem
especially interesting or important at this time.
Celotno besedilo
Dostopno za:
CMK, DOBA, FMFMET, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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