Mass-loss Mechanism Research Articles

Supernovae and their Evolution in a Low Metallicity ISM

AbstractObservations of core collapse supernovae and their progenitors generally support expectations of increasing mass loss with increasing initial mass. Mass loss rates are expected to decline at lower metallicity, and there are prospects for directly testing this for the red supergiant progenitors of Type IIP supernovae. However, there are indications that mass loss rates for high mass early type stars may be overestimated and that there are mass loss mechanisms that do not decline at lower metallicity. In this case, there may be supernova emission from strong circumstellar interaction even at low metallicity. Although there is evidence for dust formation in freely expanding ejecta of supernovae, the quantities are relatively small. Another promising site of dust formation is the circumstellar interaction region, but this should occur in only a fraction of supernovae.

On some prospects of UV observatories for studies of galaxies-IGM mass exchange

Two mechanisms of mass loss: galactic wind accompanied by the expulsion of supershell remnants and dust expelling from galaxies are briefly considered. Theoretically estimated parameters of the supershell fragments are similar to those of observed objects in vicinity of the Milky Way: small HI clouds and OIV structures observed with FUSE. Dust expelling brings dust in the IGM, and thus the analysis of intergalactic extinction curve will reveal a nature of the intergalactic dust. Evaporation of circumgalactic dust will change the chemical composition of gaseous galactic coronae. Ultraviolet observatories will have a significant impact on studies of these processes. Some prospects of such observations are outlined.

Mass loss from pulsating cool stars

AbstractIt has long been clear that most, if not all, of the mass loss experienced by stars from 0.8 to 8 solar masses occurs near the tip of the AGB and/or the RGB. Evolutionary studies have incorporated empirical mass loss laws but theoretical models suggest quite different dependence of mass loss rate on stellar parameters. We are combining evolutionary model calculations with ISUEVO with mass loss modeling using the Bowen code in a systematic study of final stages of stellar evolution. We mapped the RGB (without steady mass loss) to the “Death Zone” as a function of mixing length, mass, and metallicity. We compared these results with observation data from Origlia. We are investigating a possible mass loss mechanism through companions as a complement to mass loss through pulsation. By the end of the project we expect to provide a reliable prescription for AGB mass loss.

Three-component modelling of C-rich AGB star winds - IV. Revised interpretation with improved numerical descriptions

Models describing dust-driven winds are important for understanding the physical mechanism and properties of mass loss on the asymptotic giant branch. These models are becoming increasingly realistic with more detailed physics included, but also more computationally demanding. The purpose of this study is to clarify to what extent the applied numerical approach affects resulting physical structures of modelled winds, and to discuss resulting changes. Following the previously developed radiation hydrodynamic model – which includes descriptions for time-dependent dust formation and gas–dust drift – and using its physical assumptions and parameters, numerical improvements are introduced. Impacts of the so-called adaptive grid equation and advection schemes are assessed from models calculated with different numerical setups. Results show that wind models are strongly influenced by numerical imprecision, displaying differences in calculated physical properties of up to 100 per cent. Using a non-adaptive grid, models become periodic (in multiples of stellar pulsation periods), instead of irregular, as obtained previously. Furthermore, the numerical improvements reveal changes in physical structures. The influence of gas–dust drift is confirmed to be highly important, in particular for the dust component. Gas and dust are less tightly coupled than previously, and drastically larger amounts of dust form assuming drift.

Binary coalescence from case A evolution: mergers and blue stragglers

Abstract We have constructed some main-sequence mergers from case A binary evolution and we have studied their characteristics using the stellar evolution code written by Eggleton. The values of both total mass and orbital angular momentum are conservative in our binary evolutions. Assuming that the matter from the secondary homogeneously mixes with the envelope of the primary and that no mass is lost from the system during the merger process, we found that some mergers might be on the left of the zero-age main sequence as defined by normal surface composition (i.e. helium content Y= 0.28 with metallicity Z= 0.02 for Population I) on a colour–magnitude diagram because of enhanced surface helium content. The study also shows that the central hydrogen content of the mergers is independent of mass. Our simple models can possibly explain a few blue stragglers observed on the left of zero-age main sequence in some clusters. However, the concentration towards the blue side of the main sequence with decreasing mass, as predicted by Sandquist et al., will not appear in our models. The products with little central hydrogen in our models are probably subgiants when they are formed, as the primaries in the progenitors also have little central hydrogen and will likely leave the main sequence during the merger process. As a consequence, we fit the formula of magnitude Mv and B–V of the mergers when they return to thermal equilibrium with maximum errors 0.29 and 0.037, respectively. Taking the above into account, we performed Monte Carlo simulations to examine our models in an old open cluster NGC 2682 and an intermediate-age cluster NGC 2660. The angular momentum loss (AML) of low-mass binaries is very important in NGC 2682, and its effect can be estimated in a simple way. In NGC 2682, binary mergers from our models cover the region with high luminosity and those from the AML are located in the region with low luminosity, which has a certain width. The blue stragglers from the AML are much more numerous than those from our models, indicating that the AML of low-mass binaries makes a major contribution to blue stragglers in this old cluster. Our models can account for several blue stragglers in NGC 2660. For the most likely region on the colour–magnitude diagram, however, no blue stragglers have yet been observed. Our results are well matched with the observations if there is ∼0.5 M⊙ of mass loss in the merger process; however, it is difficult to find a physical mechanism for this much mass loss.

The Evolutionary State of the Cool Hypergiants – Episodic Mass Loss, Convective Activity and Magnetic Fields

AbstractThe evolved cool stars near the empirical upper luminosity boundary in the HR Diagram all show evidence for considerable instability perhaps due to their proximity to this stability limit and/or their evolutionary state. Recent high resolution imaging and spectroscopy of several of these stars have revealed a subset characterized by complex ejecta and evidence for episodic mass loss driven by convective activity and magnetic fields. This group includes famous stars such as the red supergiants VY CMa, NML Cyg and the post RSG IRC +10420. I will review the observational evidence and discuss the implications for the final stages of these evolved stars, their mass loss mechanism, and evolutionary state.

On the Origin of Radio Core Emission in Radio-quiet Quasars

We present a model for the radio emission from radio-quiet quasar nuclei. We show that a thermal origin for the high brightness temperature, flat spectrum point sources (known as radio cores) is possible provided that the emitting region is hot and optically thin. We hence demonstrate that optically thin bremsstrahlung from a slow, dense disk wind can make a significant contribution to the observed levels of radio core emission. This is a much more satisfactory explanation, particularly for sources where there is no evidence of a jet, than a sequence of self-absorbed synchrotron components that collectively conspire to give a flat spectrum. Furthermore, such core phenomena are already observed directly via milliarcsecond radio imaging of the Galactic microquasar SS 433 and the active galaxy NGC 1068. We contend that radio-emitting disk winds must be operating at some level in radio-loud quasars and radio galaxies as well (although in these cases, observations of the radio cores are frequently contaminated/dominated by synchrotron emission from jet knots). This interpretation of radio core emission mandates mass accretion rates that are substantially higher than Eddington. Moreover, acknowledgment of this mass-loss mechanism as an AGN feedback process has important implications for the input of energy and hot gas into the intergalactic medium (IGM) since it is considerably less directional than that from jets.

TKira—A Hybrid N-Body Code

AbstractAccurately modeling the evolution of a star cluster in a strong tidal field poses unique computational challenges. We present a hybrid code that combines the strengths of two different approaches to computing gravitational forces. The internal, collisional, dynamics of the cluster is followed with a direct N-body integrator, Kira, while the galactic tidal field is modeled with a cosmological code, GADGET, that uses a Barnes-Hut tree to evaluate gravitational forces in O(N log N) time. The quadrupole moment at the center of mass of the cluster is used to compute the external potential and provides a mechanism for mass loss. This forms a robust, bidirectional interaction. The advantages of combining two highly-developed and well-established software packages at such high level are obvious and many; not the least of the these is the ability to include other physical processes, e.g., stellar evolution. One problem to which we applied this technique is the evolution of a dense star cluster near the Galactic Center. We are also using this code to explore the effects of the strong time variation in the tidal field of merging galaxies on the evolution of young star clusters forming during the merger.

Extended Envelopes around Galactic Cepheids. III. Y Ophiuchi and α Persei from Near‐Infrared Interferometry with CHARA/FLUOR

Unbiased angular diameter measurements are required for accurate distances to Cepheids using the interferometric Baade-Wesselink method (IBWM). The precision of this technique is currently limited by interferometric measurements at the 1.5% level. At this level, the center-to-limb darkening (CLD) and the presence of circumstellar envelopes (CSE) seem to be the two main sources of bias. The observations we performed aim at improving our knowledge of the interferometric visibility profile of Cepheids. In particular, we assess the systematic presence of CSE around Cepheids in order determine accurate distances with the IBWM free from CSE biased angular diameters. We observed a Cepheid (Y Oph) for which the pulsation is well resolved and a nonpulsating yellow supergiant (α Per) using long-baseline near-infrared interferometry. We interpreted these data using a simple CSE model we previously developed. We found that our observations of α Per do not provide evidence for a CSE. The measured CLD is explained by an hydrostatic photospheric model. Our observations of Y Oph, when compared to smaller baseline measurements, suggest that it is surrounded by a CSE with characteristics similar to CSEs found previously around other Cepheids. We have determined the distance to Y Oph to be d = 491 ± 18 pc. Additional evidence points toward the conclusion that most Cepheids are surrounded by faint CSEs, detected by near-infrared interferometry: after observing four Cepheids, all show evidence for a CSE. Our CSE nondetection around a nonpulsating supergiant in the instability strip, α Per, provides confidence in the detection technique and suggests a pulsation driven mass-loss mechanism for the Cepheids.

Iron oxide and pyrocatechol: a spectroscopy study of the reaction products

The reaction of 1,2-dihydroxy-benzene (pyrocatechol) (C6H6O2) with iron oxide (Fe2O3) and sodium thiosulfate (Na2S2O3) in aqueous medium (pH 7) was investigated. Pyrocatechol suffers autoxidation and coordinates with Fe3+ in solution. The presence of S2O32- in solution was fundamental to generate and stabilize the pyrocatechol oxidation products as o-semiquinones. This compound was isolated and its structure characterized using FT-IR, EPR and UV-Vis Spectroscopy as [CTA][Fe(SQ)2(Cat)]. A thermal mass loss mechanism was proposed based on Thermogravimetric Analysis (TG) to support the structural characterization.

Properties of massive Population III and metal-poor stars

AbstractWe review the properties of massive Population III and very metal-poor stars, including briefly their formation, IMF, their main sequence evolution, possible mass loss mechanisms, atmosphere modeling etc. For detailed predictions concerning the properties of these stars we refer to Schaerer (2002) and Schaerer (2002) and references therein. Extending these calculations, Schaerer (2007) present new calculations concerning the ionizing power, Ly-α strength and related properties for different metallicities as well as for a range of power-law and log-normal IMFs. For illustrations from these studies see the Figures below. New detailed calibrations for solar metallicity O-type stars have recently been presented by Martins et al. (2005).

(Mostly) dormant comets in the NEO population and the meteoroid streams that they crumble into

AbstractMany of our annual showers do not have active parent comets. In recent years, minor planets have been identified that move among the meteoroids streams. Some streams, such as the Quadrantids, Geminids, and Sextantids, are in such unusual orbits that the probability of a chance association is only of order 1 in 106. The streams identify those objects as dormant comet nuclei. Other streams, such as the Phoenicids and α-Capricornids are associated with minor planets that were found to be weakly active at their last perihelion passage. All the streams investigated so far are young, less than 2,000 years old, and can not have been created in the classical sense of meteoroids being ejected from the comet nucleus by water vapor drag. Instead, these (mostly) dormant comets lost fragments at some point in the past, which crumbled into meteoroid streams. Scars of such events are now identified on the surface of active Jupiter family comets 9P/Tempel 1 and 81P/Wild 2. Thus, the meteor showers on Earth bear witness to what is the dominant mass-loss mechanism of comets in the inner solar system, a process that can account for much of the zodiacal cloud dust and the zodiacal dust bands.

The IRS Spitzer Spectra of the Magellanic Cloud Planetary Nebulae: Revealing the Dust and Gas Chemistry

Planetary nebulae (PNe) in the Magellanic Clouds (LMC, SMC) offer a unique opportunity to study both the population and evolution of low- and intermediate-mass stars in an environment which is free of the distance scale bias that hinders Galactic PN studies. The emission shown by PNe in the 5–40 m range is characterized by the presence of a combination of solid state features (from the dust grains) and nebular emission lines superimposed on a strong dust continuum. We acquired low resolution IRS spectroscopy of a selected sample of LMC and SMC PNe whose morphology, size, central star brightness, and chemical composition are known. The data have been acquired and reduced, and the IRS spectra show outstanding quality as well as very interesting features. The preliminary analysis presented here allows to determine strong correlations between gas and dust composition, and nebular morphology. More detailed analysis in the future will deepen our knowledge of the mass-loss mechanism, its efficiency, and its relation to PN morphology.

Dust formation and mass-loss in long-period variable stars

An approach to describe dust forming circumstellar shells around pulsating Asymptotic Giant Branch stars is presented with the aim of understanding the mass-loss mechanism of these objects. The resulting model structures are discussed and the model predictions are compared with suitable observations. Finally, some implications of the mass-loss process on the evolution of the mass-losing star are discussed.

An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars

We present an empirical determination of the mass-loss rate as a function of stellar luminosity and effective temperature, for oxygen-rich dust-enshrouded Asymptotic Giant Branch stars and red supergiants. To this aim we obtained optical spectra of a sample of dust-enshrouded red giants in the Large Magellanic Cloud, which we complemented with spectroscopic and infrared photometric data from the literature. Two of these turned out to be hot emission-line stars, of which one is a definite B[e] star. The mass-loss rates were measured through modelling of the spectral energy distributions. We thus obtain the mass-loss rate formula log Mdot = -5.65 + 1.05 log(L / 10,000 Lsun) -6.3 log(Teff / 3500 K), valid for dust-enshrouded red supergiants and oxygen-rich AGB stars. Despite the low metallicity of the LMC, both AGB stars and red supergiants are found at late spectral types. A comparison with galactic AGB stars and red supergiants shows excellent agreement between the mass-loss rate as predicted by our formula and that derived from the 60 micron flux density for dust-enshrouded objects, but not for optically bright objects. We discuss the possible implications of this for the mass-loss mechanism.

Osteoporosis, mineral metabolism, and serum soluble tumor necrosis factor receptor p55 in viral cirrhosis.

Liver cirrhosis is a risk factor for osteoporosis. Nevertheless, little is known about the mechanisms of bone mass loss in patients with viral cirrhosis. TNFalpha is a potent bone-resorbing agent. Serum concentrations of soluble TNF receptor p55 (sTNFR-55) correlate with clinical activity in liver cirrhosis. Our aim was to evaluate the possible role of sTNFR-55 in the pathogenesis of osteoporosis in patients with viral cirrhosis and its relationship with bone turnover markers. We studied 40 consecutive patients with viral cirrhosis and no history of alcohol intake and 26 healthy volunteers. Bone mineral density (BMD) was measured by dual x-ray absorptiometry in the lumbar spine (LS) and femoral neck (FN). Patients with viral cirrhosis had reduced BMD (expressed as the z-score) in all sites [LS, -1.5 +/- 0.22 (P < 0.001); FN, -0.37 +/- 0.15 (P < 0.01)]. Serum concentrations of sTNFR-55 and urinary deoxypyridinoline, a biochemical marker of bone resorption, were significantly higher in patients with osteoporosis than in patients without osteoporosis (P < 0.001 and P < 0.05, respectively). Serum levels of sTNFR-55 correlated inversely with BMD in LS (r = -0.62; P < 0.005) and FN (r = -0.47; P < 0.05) and positively with urinary deoxypyridinoline (r = 0.72, P < 0.001). Our findings show that high serum concentrations of sTNFR-55 play a role in the pathogenesis of viral cirrhosis-associated bone mass loss and provide evidence of increased bone resorption related to the high serum sTNFR-55 levels.

Preparation and Spectroscopic Characterization of Two Manganese(II)semiquinone Complexes

The complexes [CTA][Mn(II)(SQ)3] were isolated in the solid state and purified. SQ is the o-semiquinone of L-dopa or dopamine and CTA is the cetyltrimethylammonium cation. These complexes were characterized by Raman, infrared, EPR and thermogravimetry (TG) techniques. The EPR spectra of the solids presented an intense signal characteristic of the o-semiquinone radical anion with g=2.0062 and g=2.0063 for L-dopa and dopamine, respectively. Six characteristic lines around the organic radical signal confirm the presence of the Mn2+ ion. The most intense Raman bands were observed at \(\bar{\nu}\) for dopamine and at 1356 cm−1 for L-dopa and assigned to a C–O stretching with major C1–C2 character. The absence of an intense Raman band at ca. \(\bar{\nu}=1480\,{\rm cm}^{-1}\), characterizes the ligands as an o-semiquinone radical anion. Broad bands in the \(\bar{\nu}=400-750\,{\rm cm}^{-1}\) region can be assigned to deformations associated with the five-member ring chelate including the manganese ion, the oxygens, and the C1–C2 bonds. The more intense IR bands for the dopamine and the L-dopa-derived ligands at \(\bar{\nu}=1233\ {\rm and} \ 1229\, {\rm cm}^{-1}\) are assigned to \(\bar{\nu}\)CO. Mass loss mechanisms for the two complexes, based on the TG results, were proposed and confirm the formula proposed.

On the gas temperature in the shocked circumstellar envelopes of pulsating stars

In this paper, we examine the mass loss mechanism of C-rich AGB stars by means of spherically symmetric model calculations, which combine hydrodynamics, grey radiative transfer (with constant gas and variable dust opacity) and time-dependent dust formation (based on equilibrium chemistry and modified classical nucleation theory) with a time-independent non-LTE description of the state functions of the gas, in particular concerning the radiative heating and cooling function. According to our models, the dissipative heating by shock waves created by the stellar pulsation does not lead to a long-lasting increase of the gas temperatures close to the star, because the radiative cooling is too effective. The gas results to be mostly in radiative equilibrium (RE), except for some narrow but hot post-shock cooling zones. Consequently, the dust formation and wind acceleration proceeds in a similar way as described by Fleischer et al. ([CITE]) and we find a dust-driven wind triggered by the stellar pulsation, but no evidence for a purely pulsation-driven mass loss. Several new effects occur in the models which are causally connected with the non-LTE state functions. In particular, the dissociation/re-formation of H2 consumes/liberates so much energy that the radiative relaxation towards RE can be significantly delayed in regions where the phase transition takes place. These regions may stay in non-RE for a considerable fraction of the stellar pulsation period. The radiative cooling behind the strongest, dissociative shock waves ( ) usually proceeds in a two-step process where the initially rapid cooling by permitted atomic lines down to ≈ K is followed by a second phase of intense radiative cooling below ≈3000 K, as soon as the first molecules (e.g. CO) have formed. In the meantime, the gas cools slowly by forbidden metal emission lines, or by adiabatic expansion. This re-increase of the radiative cooling function with decreasing gas temperature causes a radiative instability which may temporarily lead to a coexistence of cool molecule-rich and warm molecule-poor regions in the radiative relaxation zone.

Evidence for Very Extended Gaseous Layers around O‐rich Mira Variables and M Giants

Nine bright O-rich Mira stars and five semiregular variable cool M giants have been observed with the Infrared and Optical Telescope Array (IOTA) interferometer in both K' (~2.15 μm) and L' (~3.8 μm) broadband filters, in most cases at very close variability phases. All of the sample Mira stars and four of the semiregular M giants show strong increases, from ≃20% to ≃100%, in measured uniform-disk (UD) diameters between the K' and L' bands. (A selection of hotter M stars does not show such a large increase.) There is no evidence that K' and L' broadband visibility measurements should be dominated by strong molecular bands, and cool expanding dust shells already detected around some of these objects are also found to be poor candidates for producing these large apparent diameter increases. Therefore, we propose that this must be a continuum or pseudocontinuum opacity effect. Such an apparent enlargement can be reproduced using a simple two-component model consisting of a warm (1500-2000 K), extended (up to ≃3 stellar radii), optically thin (τ ≃ 0.5) layer located above the classical photosphere. The Planck weighting of the continuum emission from the two layers will suffice to make the L' UD diameter appear larger than the K' UD diameter. This two-layer scenario could also explain the observed variation of Mira UD diameters versus infrared wavelength—outside of strong absorption bands—as already measured inside the H, K, L, and N atmospheric windows. This interpretation is consistent with the extended molecular gas layers (H2O, CO, etc.) inferred around some of these objects from previous IOTA K'-band interferometric observations obtained with the Fiber Linked Unit for Optical Recombination (FLUOR) and from Infrared Space Observatory and high-resolution ground-based FTS infrared spectra. The two-component model has immediate implications. For example, the Mira photosphere diameters are smaller than previously recognized—this certainly implies higher effective temperatures, and it may favor fundamental mode pulsation. Also, the UD model fails generally to represent the brightness distribution and has very limited applicability for Mira stars. The presence of a very extended gas layer extending up to ≃3 stellar radii seems now well established on a fair sample of asymptotic giant branch stars ranging from late-type giants to long-period variables, with some probable impact on stellar model atmospheres and mass-loss mechanisms.

The timing of initiation of fast-flowing ice streams during a glacial cycle inferred from glacimarine sedimentation

Fast-flowing ice streams drain huge basins within modern ice sheets and are a mechanism for rapid iceberg production and mass loss. Some ice streams are known to be unstable, and to switch between fast flow and stagnation. Numerical ice-sheet models and geophysical observations of large-scale streamlined glacial landforms and large glacier-derived sedimentary fans along high-latitude continental margins indicate the presence of ice streams in Quaternary ice sheets. High-resolution AMS dated sedimentary records from an inter-fan area on the Svalbard continental margin yield evidence on the timing of initiation of fast glacier flow during a glacial cycle of ice-sheet growth and decay. Sedimentation rates and iceberg-rafted debris influx show that debris delivery to the Svalbard margin was relatively high during the period of Late Weichselian ice-sheet growth from about 30 000–18 000 yr BP. Both parameters fell dramatically in inter-fan areas when full-glacial conditions, with ice at the continental shelf edge, were established at about 18 000 yr BP. This shift is interpreted to mark the initiation of a fast-flowing ice stream, the convergence of flow from the surrounding ice sheet into this ice stream, and the consequent slowing of ice flow and sediment delivery to the inter-fan area beyond the ice-stream margins. This provides a constraint on the timing of ice-stream initiation during the cycle of ice-sheet growth and decay.