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Roberta M. Humphreys

Publications and source records attributed to Roberta M. Humphreys.

At least 19 recordsLinked to original sources

On the Spatial Distribution of Luminous Blue Variables, B[e] Supergiants, and Wolf-Rayet stars in the Large Magellanic Cloud

We examine the spatial distributions of LBVs, B[e] supergiants, and W-R stars in the LMC, to clarify their relative ages, evolutionary states, and relationships. This survey employs a reference catalog that was not available for previous work, comprising more than 3900 of the LMC's most luminous stars. Our analysis shows that LBVs, B[e] supergiants, and WR's have spatial distributions like normal stars with the same spectral types and luminosities. Most LBVs are not isolated, nor do they require binary or multiple status to explain their spatial relationship to other populations. There are two likely exceptions: one lower-luminosity LBV and one LBV candidate are relatively isolated and may have velocities that require additional acceleration. The B[e] supergiants are spatially and kinematically more dispersed than LBVs, suggesting that they belong to an older population. The most luminous early-type WN's are most closely associated with the evolved late O-type supergiants. The high luminosity late WNs, and WNh stars however, are highly concentrated in the 30 Dor region which biases the analysis. The less luminous WNs and WCs are associated with a mix of evolved late B, A-type, and yellow supergiants which may be in a post-red-supergiant phase. Spatial distributions of the less luminous WN, WC, and WN3/O3 stars reinforce proposed evolutionary links among those subtypes. Our analysis also demonstrates the importance of using a comprehensive census, with reference populations clearly defined by spectral type and luminosity, and how small number statistics, especially combined with spatial clustering, can invalidate some commonly-cited statistical tests.

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Red Supergiants -- The Other Side of the H-R Diagram

Red supergiants are the largest stars known with some of the highest mass loss rates observed. They are the final stage in the evolution of the majority of massive stars. The unexpected discovery of high mass loss episodes in many red supergiants have posed questions about the role of mass loss on their final stages. The papers in this volume are timely reviews of our current understanding of this often surprising population of massive stars. This introductory paper is a brief summary of their observed properties and a historical perspective on some of the current problems on mass loss, their circumstellar environments, and their evolutionary state.

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The Infrared-Bright SW Knots in the Complex Ejecta of VY CMa

The red hypergiant VY CMa is remarkable for its very visible record of high mass loss events observed over the range of wavelengths from the optical and infrared to the submillimeter region with ALMA. The SW Clump or SW knots are unique in the ejecta of VY CMa. Except for the central star, they are the brightest sources of dusty infrared emission in its complex ejecta. In this paper we combine the proper motions from the HST images, and infrared fluxes from 2 to 12 microns with the 12CO images from ALMA to determine their ages and mass estimates. The SW knots were ejected more than 200 years ago with an active period lasting about 30 years, and with a total mass in the Clump more than 0.02 Solar masses.

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The Hidden Clumps in VY CMa Uncovered by ALMA

The red hypergiant VY CMa is famous for its very visible record of high mass loss events. Recent CO observations with ALMA revealed three previously unknown large scale outflows (Paper I). 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 HST, VY CMa experienced at least six massive outflows during a 30 year period with a total mass lost greater than 0.07 Msun. 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.

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Yellow Supergiants and Post-Red Supergiant Evolution in the Large Magellanic Cloud

The empirical evidence for an upper mass limit for the red supergiant (RSG) progenitors of the Type II-P SNe at about 18 Msun, raises questions about the fate of the most luminous, most massive RSGs. These stars may evolve back to warmer temperatures to end their lives as hotter stars or collapse directly to black holes. The yellow hypergiants, many with extensive circumstellar dust and high mass loss, are excellent candidates for post-RSG evolution. We have identified six high luminosity yellow supergiants (YSGs) in the LMC with circumstellar dust including two of the FYPS (Dorn et al, 2022). We discuss their SEDs, mass lost and mass loss rates. Together with three additional FYPS, these nine stars are about 1/3 of the YSGs above 10^5 Lsun. We conclude that the high luminosity YSGs with surface pulsations and circumstellar dust, distinct from other YSGs, are candidates for post-RSG evolution in the LMC.

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A New LBV Candidate in M33

The evolutionary relationships and mechanisms governing the behavior of the wide variety of luminous stars populating the upper H-R diagram are not well established. Luminous blue variables (LBVs) are particularly rare, with only a few dozen identified in the Milky Way and nearby galaxies. Since 2012, the Barber Observatory Luminous Stars Survey has monitored more than 100 luminous targets in M33, including M33C-4119 which has recently undergone photometric and spectroscopic changes consistent with an S Doradus eruption of an LBV.

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Episodic Gaseous Outflows and Mass Loss from Red Supergiants

The red hypergiant VY CMa and the more typical red supergiant Betelgeuse provide clear observational evidence for discrete, directed gaseous outflows in their optical and infrared imaging, spectra, and light curves. In the very luminous VY CMa, mass loss estimates from the infrared bright knots and clumps, not only dominate its measured overall mass loss, but explain it. In the lower luminosity Betelgeuse, similar mass estimates of its circumstellar condensations show that they contribute significantly to its measured mass loss rate. We present new measurements for both stars and discuss additional evidence for gaseous ejections in other red supergiants. Gaseous outflows are the dominant mass loss mechanism for the most luminous RSGs and an important contributor to the more typical red supergiants like Betelgeuse. We conclude that gaseous outflows, related to magnetic fields and surface activity, comparable to coronal mass ejections, are a major contributor to mass loss from red supergiants and the missing component in discussions of their mass loss mechanism.

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The Altered State of $η$Carinae: HST's Photometric Record 1998--2021

Hubble Space Telescope photometry of $η$ Carinae spans 23 years, including five spectroscopic events. The rapid brightening rate decreased after 2010, and the spectroscopic events in 2014 and 2020 had light curves different from their predecessors. Together with other indicators, these developments probably foretell the conclusion of $η$ Car's change of state.

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The Mass-Loss History of the Red Hypergiant VY CMa

Imaging and spectroscopy of the knots, clumps, and extended arcs in the complex ejecta of VY CMa confirm a record of high mass loss events over the past few hundred years. HST/STIS spectroscopy of numerous small knots close to the star allow us to measure their radial velocities from the strong K I emission and determine their separate motions, spatial orientations, and time since ejecta. Their ages concentrate around 70, 120, 200 and 250 years ago. A K I emission knot only 50 mas from the star ejected as recently as 1985 -- 1995 may coincide with an H2O maser. Comparison with VY CMa's historic light curve from 1800 to the present, shows several knots with ejection times that correspond with extended periods of variability and deep minima. The similarity of this correspondence in VY CMa with the remarkable recent dimming of Betelgeuse and an outflow of gas is apparent. The evidence for similar outflows from the surface of a more typical red supergiant suggests that discrete ejections are more common and surface or convective activity is a major source of mass loss for red supergiants.

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Red Supergiants, Yellow Hypergiants, and Post-RSG Evolution

How massive stars end their lives remains an open question in the field of star evolution. While the majority of stars above 9 M_sun will become red supergiants (RSGs), the terminal state of these massive stars can be heavily influenced by their mass-loss histories. Periods of enhanced circumstellar wind activity can drive stars off the RSG branch of the HR Diagram. This phase, known as post-RSG evolution, may well be tied to high mass-loss events or eruptions as seen in the Luminous Blue Variables and other massive stars. This article highlights some of the recent observational and modeling studies that seek to characterize this unique class of stars, the post-RSGs, and link them to other massive objects on the HR Diagram such as LBVs, Yellow Hypergiants, and dusty RSGs.

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The Complex Upper HR Diagram

Several decades of observations of the most massive and most luminous stars have revealed a complex upper HR Diagram, shaped by mass loss, and inhabited by a variety of evolved stars exhibiting the consequences of their mass loss histories. This review presents a brief historical overview of the HR Diagram for massive stars, highlighting some of the primary discoveries and results from their observation in nearby galaxies. The chapters in this volume include reviews of our current understanding of different groups of evolved massive stars, all losing mass and in different stages of their evolution; the Luminous Blue Variables (LBVs), B[e] supergiants, the warm hypergiants, Wolf-Rayet stars, and the population of OB stars and supergiants in the Magellanic Clouds.

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Exploring the Mass Loss Histories of the Red Supergiants

We report mid- to far-infrared imaging and photomety from 7 to 37 microns with SOFIA/FORCAST and 2 micron adaptive optics imaging with LBTI/LMIRCam of a large sample of red supergiants (RSGs) in four Galactic clusters; RSGC1, RSGC2=Stephenson 2, RSGC3, and NGC 7419. The red supergiants in these clusters cover their expected range in luminosity and initial mass from approximately 9 to more than 25 Solar masses. The population includes examples of very late-type RSGs such as MY Cep which may be near the end of the RSG stage, high mass losing maser sources, yellow hypergiants and post-RSG candidates. Many of the stars and almost all of the most luminous have spectral energy distributions (SEDs) with extended infrared excess radiation at the longest wavelengths. To best model their SEDs we use DUSTY with a variable radial density distribution function to estimate their mass loss rates. Our mass loss rate -- luminosity relation for 42 RSGs basically follows the classical de Jager curve, but at luminosities below 10^5 Solar luminosities we find a significant population of red supergiants with mass loss rate below the de Jager relation. At luminosities above 10^5 Solar luminosities there is a rapid transition to higher mass loss rates that approximates and overlaps the de Jager curve. We recommend that instead of using a linear relation or single curve, the empirical mass loss rate -- luminosity relation is better represented by a broad band. Interestingly, the transition to much higher mass loss rates at about 10^5 Lsun corresponds approximately to an initial mass of 18 --20 Msun which is close to the upper limit for RSGs becoming Type II SNe.

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Thermal Emission in the Southwest Clump of VY CMa

We present high spatial resolution LBTI/NOMIC $9-12$ $μm$ images of VY CMa and its massive outflow feature, the Southwest (SW) Clump. Combined with high-resolution imaging from HST ($0.4-1$ $μm$) and LBT/LMIRCam ($1-5$ $μm$), we isolate the spectral energy distribution (SED) of the clump from the star itself. Using radiative-transfer code DUSTY, we model both the scattered light from VY CMa and the thermal emission from the dust in the clump to estimate the optical depth, mass, and temperature of the SW Clump. The SW Clump is optically thick at 8.9 $μm$ with a brightness temperature of $\sim$200 K. With a dust chemistry of equal parts silicates and metallic iron, as well as assumptions on grain size distribution, we estimate a dust mass of $5.4\times10^{-5}\,M_\odot$. For a gas--to--dust ratio of 100, this implies a total mass of $5.4\times10^{-3}\,M_\odot$. Compared to the typical mass-loss rate of VY CMa, the SW Clump represents an extreme, localized mass-loss event from $\lesssim300$ years ago.

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Comments on the Progenitor of NGC6946-BH1

The broad-band colors of the progenitor of the black hole candidate in NGC 6946 suggest that it was a yellow hypergiant on a post-red supergiant track when the core collapse occurred.

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The Unexpected Spectrum of the Innermost Ejecta of the Red Hypergiant VY CMa

HST/STIS spectra of the small clumps and filaments closest to the central star in VY CMa reveal that the very strong K I emission and TiO and VO molecular emission, long thought to form in a dusty circumstellar shell, actually originate in a few small clumps 100's of AU from the star. The K I lines are 10 to 20 times stronger in these nearest ejecta than on the star. The observations also confirm VO as a circumstellar molecule. In this letter we discuss the spectra of the features, their motions and ages, and the identification of the molecular emission. The strength of the atomic and molecular features in the small clumps present an astrophysical problem for the excitation process. We show that the clumps must have a nearly clear line of sight to the star's radiation.

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Luminous and Variable Stars in NGC 2403 and M81

We present the results of spectroscopy and multi-wavelength photometry of luminous and variable star candidates in the nearby spiral galaxies NGC 2403 and M81. We discuss specific classes of stars, the Luminous Blue Variables (LBVs), B[e] supergiants (sgB[e]), and the high luminosity yellow hypergiants. We identify two new LBV candidates, and three sgB[e] stars in M81. We also find that some stars previously considered LBV candidates are actually field stars. The confirmed and candidate LBVs and sgB[e] stars together with the other confirmed members are shown on the HR Diagrams for their respective galaxies. We also present the HR Diagrams for the two "SN impostors", V37 (SN2002kg) and V12(SN1954J) in NGC 2403 and the stars in their immediate environments.

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Gaia, Trumpler 16, and Eta Carinae

Gaia parallaxes for the star cluster Tr 16 reveal a discrepancy in the oft-quoted distance of Eta Carinae. It is probably more distant and more luminous. Moreover, many presumed members may not belong to Tr 16.

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Eta Carinae's Declining Outflow Seen in the UV, 2002-2015

Existing HST UV data offer many previously neglected clues to eta 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] 1750 is one of the two most luminous emission features in eta Car's observable spectrum, comparable to H-alpha. 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 eta 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. 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.

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