Pulsation-Initiated Mass Loss in Luminous Blue Variables: A Parameter Study

Luminous blue variables (LBVs) are characterized by semi-periodic episodes of enhanced mass-loss, or outburst. The cause of these outbursts has thus far been a mystery. One explanation is that they are initiated by kappa-effect pulsations in the atmosphere caused by an increase in luminosity at temperatures near the so-called ``iron bump'' (T ~ 200,000 K), where the Fe opacity suddenly increases. Due to a lag in the onset of convection, this luminosity can build until it exceeds the Eddington limit locally, seeding pulsations and possibly driving some mass from the star. We present some preliminary results from a parameter study focusing on the conditions necessary to trigger normal S-Dor type (as opposed to extreme eta-Car type) outbursts. We find that as Y increases or Z decreases, the pulsational amplitude decreases and outburst-like behavior, indicated by a large, sudden increase in photospheric velocity, becomes likes likely.Comment: 6 pages, 4 figures, to be published in the Proceedings of Massive Stars as Cosmic Engines, IAU Symp 250, ed. F. Bresolin, P. A. Crowther, & J. Puls (Cambridge Univ. Press

The yellow hypergiants HR 8752 and rho Cassiopeiae near the evolutionary border of instability

High-resolution near-ultraviolet spectra of the yellow hypergiants HR 8752 and rho Cassiopeiae indicate high effective temperatures placing both stars near the T_eff border of the ``yellow evolutionary void''. At present, the temperature of HR 8752 is higher than ever. For this star we found Teff=7900+-200 K, whereas rho Cassiopeiae has Teff=7300+-200 K. Both, HR 8752 and rho Cassiopeiae have developed strong stellar winds with Vinf ~ 120 km/s and Vinf ~ 100 km/s, respectively. For HR 8752 we estimate an upper limit for the spherically symmetric mass-loss of 6.7X10^{-6}M_solar/yr. Over the past decades two yellow hypergiants appear to have approached an evolutionary phase, which has never been observed before. We present the first spectroscopic evidence of the blueward motion of a cool super/hypergiant on the HR diagram.Comment: 13 pages including 3 figures. Accepted for publication in ApJ Letter

On the Dynamic Stability of Cool Supergiant Atmospheres

We have developed a new formalism to compute the thermodynamic coefficient Gamma1 in the theory of stellar and atmospheric stability. We generalize the classical derivation of the first adiabatic index, which is based on the assumption of thermal ionization and equilibrium between gas and radiation temperature, towards an expression which incorporates photo-ionization due to radiation with a temperature T_rad different from the local kinetic gas temperature.Our formalism considers the important non-LTE conditions in the extended atmospheres of supergiant stars. An application to the Kurucz grid of cool supergiant atmospheres demonstrates that models with T_rad =~ T_eff between 6500 K and 7500 K become most unstable against dynamic perturbations, according to Ledoux' stability integral . This results from Gamma1 and acquiring very low values, below 4/3, throughout the entire stellar atmosphere, which causes very high gas compression ratios around these effective temperatures. Based on detailed NLTE-calculations, we discuss atmospheric instability of pulsating massive yellow supergiants, like the hypergiant rho Cas (Ia+), which exist in the extension of the Cepheid instability strip, near the Eddington luminosity limit.Comment: 54 pages including figures and the Appendix, 7 figures, Accepted for The Astrophysical Journal, Main Journal, 558, Sept. 200

Mass-luminosity relation and pulsational properties of Wolf-Rayet stars

Evolution of Population I stars with initial masses from 70M_\odot to 130M_\odot is considered under various assumptions on the mass loss rate \dot M. The mass-luminosity relation of W-R stars is shown to be most sensitive to the mass loss rate during the helium burning phase \dot M_{3\alpha}. Together with the mass-luminosity relation obtained for all evolutionary sequences several more exact relations are determined for the constant ratio f_{3\alpha}=\dot M/\dot M_{3\alpha} with 0.5 \le f_{3\alpha} \le 3. Evolutionary models of W-R stars were used as initial conditions in hydrodynamic computations of radial nonlinear stellar oscillations. The oscillation amplitude is larger in W-R stars with smaller initial mass or with lower mass loss rate due to higher surface abundances of carbon and oxygen. In the evolving W-R star the oscillation amplitude decreases with decreasing stellar mass M and for M < 10M_\odot the sufficiently small nonlinear effects allow us to calculate the integral of the mechanical work W done over the pulsation cycle in each mass zone of the hydrodynamical model. The only positive maximum on the radial dependence of W is in the layers with temperature of T\sim 2e5K where oscillations are excited by the iron Z--bump kappa-mechanism. Radial oscillations of W-R stars with mass of M > 10M_\odot are shown to be also excited by the kappa-mechanism but the instability driving zone is at the bottom of the envelope and pulsation motions exist in the form of nonlinear running waves propagating outward from the inner layers of the envelope.Comment: 15 pages, 10 figures, submitted to Astronomy Letter

MEN-2 Syndrome: The Value of Screening and Central Registration; A Study of Six Kindreds in The Netherlands

Since 1975, six families with the MEN-2A syndrome including 66 patients have been identified in The Netherlands. All these patients underwent thyroidectomy for C-cell hyperplasia and/or medullary thyroid carcinoma (MTC); eight were symptomatic (Group A), 51 were relatives of patients found to be affected (Group B), and seven had had a negative screening test that became positive (Group C). To assess the effect of screening, we compared these groups with respect to the occurrence of metastatic MTC at thyroidectomy and the results of the postoperative calcitonin (CT) tests. We found that 87% of Group A, 37% of Group B; and none of Group C had metastatic disease at surgery. The cure rates in these three groups with MEN-2A, as determined by stimulated CT measurement, was 0%, 51%, and 100%, respectively. From these results it may be concluded that screening can lead to the detection of MTC at an earlier stage which in turn could permit curative treatment and improvement of both prognosis and life expectancy. The need for supervision of affected families by central registration to guarantee the continuity of screening is stressed

On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars

We suggest that the mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds. In order for a massive star to become a WR star, it must shed its H envelope, but new estimates of the effects of clumping in winds indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 Msun, for which the best alternative is mass loss during brief eruptions of luminous blue variables (LBVs). Our clearest example of this phenomenon is the 19th century outburst of eta Car, when the star shed 12-20 Msun or more in less than a decade. Other examples are circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova impostors''), and massive shells around SNe and GRBs. We do not yet fully understand what triggers LBV outbursts, but they occur nonetheless, and present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuum-driven wind or a hydrodynamic explosion, both of which are insensitive to metallicity. As such, eruptive mass loss could have played a pivotal role in the evolution and fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would profoundly affect the chemical yield and types of remnants from early SNe and hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter

Risk Factors for Failure of Direct Oral Feeding Following a Totally Minimally Invasive Esophagectomy

Càncer d'esòfag; Jejunostomia; NutricióCáncer de esófago; Yeyunostomía; NutriciónEsophageal cancer; Jejunostomy; NutritionRecently, it has been shown that directly starting oral feeding (DOF) from postoperative day one (POD1) after a totally minimally invasive Ivor-Lewis esophagectomy (MIE-IL) can further improve postoperative outcomes. However, in some patients, tube feeding by a preemptively placed jejunostomy is necessary. This single-center cohort study investigated risk factors associated with failure of DOF in patients that underwent a MIE-IL between October 2015 and April 2021. A total of 165 patients underwent a MIE-IL, in which DOF was implemented in the enhanced recovery after surgery program. Of these, 70.3% (n = 116) successfully followed the nutritional protocol. In patients in which tube feeding was needed (29.7%; n = 49), female sex (compared to male) (OR 3.5 (95% CI 1.5–8.1)) and higher ASA scores (III + IV versus II) (OR 2.2 (95% CI 1.0–4.8)) were independently associated with failure of DOF for any cause. In case of failure, this was either due to a postoperative complication (n = 31, 18.8%) or insufficient caloric intake on POD5 (n = 18, 10.9%). In the subgroup of patients with complications, higher ASA scores (OR 2.8 (95% CI 1.2–6.8)) and histological subtypes (squamous-cell carcinoma versus adenocarcinoma and undifferentiated) (OR 5.2 (95% CI 1.8–15.1)) were identified as independent risk factors. In the subgroup of patients with insufficient caloric intake, female sex was identified as a risk factor (OR 5.8 (95% CI 2.0–16.8)). Jejunostomy-related complications occurred in 17 patients (10.3%). In patients with preoperative risk factors, preemptively placing a jejunostomy may be considered to ensure that nutritional goals are met.The previous NUTRIENT II trial was funded by KWF Kankerbestrijding (Dutch Cancer Society, grant number 10495) and Medtronic (20130529)

Instability of LBV-stars against radial oscillations

In this study we consider the nonlinear radial oscillations exciting in LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <= 49.1 M_odot. Hydrodynamic computations were carried out with initial conditions obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show instability against radial oscillations with amplitude growth time comparable with dynamical time scale of the star. Radial oscillations exist in the form of nonlinear running waves propagating from the boundary of the compact core to the upper boundary of the hydrodynamical model. The velocity amplitude of outer layers is of several hundreds of km/s while the bolometric light amplitude does not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism and are due to the instability of the gas with adiabatic exponent close to the critical value Gamma_1 = 4/3 due to the large contribution of radiation in the total pressure. The range of the light variation periods (6 day <= P <= 31 day) of hydrodynamical models agrees with periods of microvariability observed in LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter

The Missing Luminous Blue Variables and the Bistability Jump

We discuss an interesting feature of the distribution of luminous blue variables on the H-R diagram, and we propose a connection with the bistability jump in the winds of early-type supergiants. There appears to be a deficiency of quiescent LBVs on the S Dor instability strip at luminosities between log L/Lsun = 5.6 and 5.8. The upper boundary, is also where the temperature-dependent S Dor instability strip intersects the bistability jump at about 21,000 K. Due to increased opacity, winds of early-type supergiants are slower and denser on the cool side of the bistability jump, and we postulate that this may trigger optically-thick winds that inhibit quiescent LBVs from residing there. We conduct numerical simulations of radiation-driven winds for a range of temperatures, masses, and velocity laws at log L/Lsun=5.7 to see what effect the bistability jump should have. We find that for relatively low stellar masses the increase in wind density at the bistability jump leads to the formation of a modest to strong pseudo photosphere -- enough to make an early B-type star appear as a yellow hypergiant. Thus, the proposed mechanism will be most relevant for LBVs that are post-red supergiants. Yellow hypergiants like IRC+10420 and rho Cas occupy the same luminosity range as the ``missing'' LBVs, and show apparent temperature variations at constant luminosity. If these yellow hypergiants do eventually become Wolf-Rayet stars, we speculate that they may skip the normal LBV phase, at least as far as their apparent positions on the HR diagram are concerned.Comment: 20 pages, 4 figs, accepted by Ap