Evolution Of Intermediate-mass Stars Research Articles

Discovery of a New Massive ZZ Ceti Star

Massive pulsating white dwarf stars are extremely rare, because they are the final product of intermediate-mass stars, which are less common than low-mass stars. Additionally, their small size makes them fainter than the normal-mass white dwarf stars (∼0.6 M ⊙). Our motivation to look for this type of variable is to be able to study in detail their internal structure and therefore derive properties for the outcome of the evolution of intermediate mass stars, below 10 M ⊙. Using the 2.1 m Otto Struve Telescope at McDonald Observatory, we report on the discovery of a new massive pulsating white dwarf star. These stars might be massive enough that their cores have a significant crystallized portion, up to about 50%. A detailed asteroseismic study of these stars will provide important constrains on intermediate-mass stellar evolution, and the opportunity to study solid state physics at extreme conditions.

Elimination of the Blue Loops in the Evolution of Intermediate-mass Stars by the Neutrino Magnetic Moment and Large Extra Dimensions

Abstract For searching beyond Standard Model physics, stars are laboratories that complement terrestrial experiments. Massless neutrinos in the Standard Model of particle physics cannot have a magnetic moment, but massive neutrinos have a finite magnetic moment in the minimal extension of the Standard Model. Large extra dimensions (LEDs) are a possible solution of the hierarchy problem. Both of these provide additional energy-loss channels in stellar interiors via the electromagnetic interaction and radiation into extra dimensions, respectively, and thus affect stellar evolution. We perform simulations of stellar evolution with such additional energy losses and find that they eliminate the blue loops in the evolution of intermediate-mass stars. The existence of Cepheid stars can be used to constrain the neutrino magnetic moment (NMM) and LEDs. In order for Cepheids to exist, the NMM should be smaller than the range ∼2 × 10−10 μ B–4 × 10−11 μ B, where μ B is the Bohr magneton, and the fundamental scale in the (4+2)-spacetime should be larger than ∼2–5 TeV, depending on the rate of the 12C reaction. The fundamental scale also has strong dependence on the metallicity. This value of the magnetic moment is in the range explored in the reactor experiments, but higher than the limit inferred from globular clusters. Similarly the fundamental scale value we constrain corresponds to a size of the compactified dimensions comparable to those explored in the torsion balance experiments, but it is smaller than the limits inferred from collider experiments and low-mass stars.

Dynamical masses of Cepheids from the GAIA parallaxes

AbstractThe mass of a Cepheid is a fundamental parameter for studying the pulsation and evolution of intermediate-mass stars. But determining this variable has been a long-standing problem for decades. Detecting the companions (by spectroscopy or imaging) is a difficult task because of the brightness of the Cepheids and the close orbit of the components. So most of the Cepheid masses are derived using stellar evolution or pulsation modeling, but they differ by 10-20 %. Measurements of dynamical masses offer the unique opportunity to make progress in resolving this mass discrepancy.The first problem in studying binary Cepheids is the high contrast between the components for wavelengths longer than 0.5 μm, which make them single-line spectroscopic binaries. In addition, the close orbit of the companions (<40 mas) prevents us from spatially resolving the systems with a single-dish 8m-class telescope. A technique able to reach high spatial resolution and high-dynamic range is long-baseline interferometry. We have started a long-term program that aims at detecting, monitoring and characterizing physical parameters of the Cepheid companions. The GAIA parallaxes will enable us to combine interferometry with single-line velocities to provide unique dynamical mass measurements of Cepheids.

KIC 10080943: An eccentric binary system containing two pressure- and gravity-mode hybrid pulsators

Gamma Doradus and delta Scuti pulsators cover the transition region between low mass and massive main-sequence stars, and as such, are critical for testing stellar models. When they reside in binary systems, we can combine two independent methods to derive critical information, such as precise fundamental parameters to aid asteroseismic modelling. In the Kepler light curve of KIC10080943, clear signatures of gravity- and pressure-mode pulsations have been found. Ground-based spectroscopy revealed this target to be a double-lined binary system. We present the analysis of four years of Kepler photometry and high-resolution spectroscopy to derive observational constraints with which to evaluate theoretical predictions of the stellar structure and evolution for intermediate-mass stars. We used the method of spectral disentangling to determine atmospheric parameters for both components and derive the orbital elements. With PHOEBE, we modelled the ellipsoidal variation and reflection signal of the binary in the light curve and used classical Fourier techniques to analyse the pulsation modes. We show that the eccentric binary system KIC10080943 contains two hybrid pulsators with masses $M_1=2.0\pm0.1~M_\odot$ and $M_2=1.9\pm0.1~M_\odot$, with radii $R_1=2.9\pm0.1~R_\odot$ and $R_2=2.1\pm0.2~R_\odot$. We detect rotational splitting in the g modes and p modes for both stars and use them to determine a first rough estimate of the core-to-surface rotation rates for the two components, which will be improved by future detailed seismic modelling.

Binary Cepheids from optical interferometry

Classical Cepheid stars have been considered since more than a century as reliable tools to estimate distances in the universe thanks to their Period-Luminosity (P-L) relationship. Moreover, they are also powerful astrophysical laboratories, providing fundamental clues for studying the pulsation and evolution of intermediate-mass stars. When in binary systems, we can investigate the age and evolution of the Cepheid, estimate the mass and distance, and constrain theoretical models. However, most of the companions are located too close to the Cepheid (1-40 mas) to be spatially resolved with a 10-meter class telescope. The only way to spatially resolve such systems is to use long-baseline interferometry. Recently, we have started a unique and long-term interferometric program that aims at detecting and characterizing physical parameters of the Cepheid companions, with as main objectives the determination of accurate masses and geometric distances.

From bipolar to elliptical: simulating the morphological evolution of planetary nebulae

The majority of Proto-planetary nebulae (PPN) are observed to have bipolar morphologies. The majority of mature PN are observed to have elliptical shapes. In this paper we address the evolution of PPN/PN morphologies attempting to understand if a transition from strongly bipolar to elliptical shape can be driven by changes in the parameters of the mass loss process. To this end we present 2.5D hydrodynamical simulations of mass loss at the end stages of stellar evolution for intermediate mass stars. We track changes in wind velocity, mass loss rate and mass loss geometry. In particular we focus on the transition from mass loss dominated by a short duration jet flow (driven during the PPN phase) to mass loss driven by a spherical fast wind (produced by the central star of the PN). We address how such changes in outflow characteristics can change the nebula from a bipolar to an elliptical morphology. Our results show that including a period of jet formation in the temporal sequence of PPN to PN produces realistic nebular synthetic emission geometries. More importantly such a sequence provides insight, in principle, into the apparent difference in morphology statistics characterizing PPN and PN systems. In particular we find that while jet driven PPN can be expected to be dominated by bipolar morphologies, systems that begin with a jet but are followed by a spherical fast wind will evolve into elliptical nebulae. Furthermore, we find that spherical nebulae are highly unlikely to ever derive from either bipolar PPN or elliptical PN.

THE YOUNG OPEN CLUSTER BERKELEY 55

We present UBV photometry of the highly reddened and poorly studied open cluster Berkeley 55, revealing an important population of B-type stars and several evolved stars of high luminosity. Intermediate resolution far-red spectra of several candidate members confirm the presence of one F-type supergiant and six late supergiants or bright giants. The brightest blue stars are mid-B giants. Spectroscopic and photometric analyses indicate an age 50+-10 Myr. The cluster is located at a distance d~4kpc, consistent with other tracers of the Perseus Arm in this direction. Berkeley 55 is thus a moderately young open cluster with a sizable population of candidate red (super)giant members, which can provide valuable information about the evolution of intermediate-mass stars.

MODULES FOR EXPERIMENTS IN STELLAR ASTROPHYSICS (MESA)

Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source libraries for a wide range of applications in computational stellar astrophysics. A newly designed 1-D stellar evolution module, MESA star, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very-low mass to massive stars, including advanced evolutionary phases. MESA star solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. Independently usable modules provide equation of state, opacity, nuclear reaction rates, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own public interface. Examples include comparisons to other codes and show evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the evolution of intermediate mass stars through the thermal pulses on the He-shell burning AGB phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; evolutionary tracks of massive stars from the pre-main sequence to the onset of core collapse; stars undergoing Roche lobe overflow; and accretion onto a neutron star. Instructions for downloading and installing MESA can be found on the project web site (http://mesa.sourceforge.net/).

DIRECT DETECTION OF THE CLOSE COMPANION OF POLARIS WITH THEHUBBLE SPACE TELESCOPE

Polaris, the nearest and brightest classical Cepheid, is a single-lined spectroscopic binary with an orbital period of 30 yr. Using the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) at a wavelength of ~2255 A, we have directly detected the faint companion at a separation of 017. A second HST observation 1.04 yr later confirms orbital motion in a retrograde direction. By combining our two measures with the spectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5 proper motions by Wielen et al., we find a mass for Polaris Aa of 4.5+2.2 –1.4 M ☉—the first purely dynamical mass determined for any Cepheid. For the faint companion Polaris Ab we find a dynamical mass of 1.26+0.14 –0.07 M ☉, consistent with an inferred spectral type of F6 V and with a flux difference of 5.4 mag observed at 2255 A. The magnitude difference at the V band is estimated to be 7.2 mag. Continued HST observations will significantly reduce the mass errors, which are currently still too large to provide critical constraints on the roles of convective overshoot, mass loss, rotation, and opacities in the evolution of intermediate-mass stars. Our astrometry, combined with two centuries of archival measurements, also confirms that the well-known, more distant (18'') visual companion, Polaris B, has a nearly common proper motion with that of the Aa, Ab pair. This is consistent with orbital motion in a long-period bound system. The ultraviolet brightness of Polaris B is in accordance with its known F3 V spectral type if it has the same distance as Polaris Aa, Ab.

Planetary nebulae and ALMA

Our understanding of the late evolution of intermediate mass stars (∼1–8M⊙) through the planetary nebula phase is undergoing major developments. Observations at infrared and millimeter wavelengths have revealed important components of neutral gas and dust in the nebulae that directly trace their formation from mass-loss on the Asymptotic Giant Branch. At the same time, high resolution imaging, especially with the Hubble Space Telescope, has revealed a surprising array of structures in the nebulae: multiple arcs, tori, jets, and myriads of small scale fragments. None of these are fully understood, and all involve the neutral gas component. This paper highlights recent observations of these structures and discusses the open questions, with an emphasis on those areas where observations with ALMA are likely to make important contributions.

On the effect of overshooting as predicted by the modelling of the pre-main-sequence evolution of a 2 M⊙ star

We discuss the effects of convective overshooting in the pre-main-sequence (PMS) evolution of intermediate-mass stars, by analysing in detail the early evolution towards the main sequence of a 2 Mstellar model. These effects can be extremely important in the end of the PMS, when the abundances in CNO elements approach the equilibrium in the centre. We provide a possible physical explanation on why a moderate amount of overshooting produces, as the star approaches the zero-age main-sequence, an extra loop in the evolutionary tracks on the Hertzsprung-Russell diagram. An interesting feature is that there is a very well defined amount of overshooting (for a given stellar mass and chemical composition) beyond which a loop is produced. For smaller amounts of overshooting such a loop does not take place and the evolutionary tracks are similar to those found in the literature. The amount of overshooting needed to produce the loop decreases with stellar mass. We discuss the underlining physical reasons for the behaviour predicted by the evolution models and argue that it provides a crucial observational test for convective overshooting in the core of intermediate-mass stars.

Diffusive convective overshoot in core He-burning intermediate mass stars

We present detailed evolutionary calculations focused on the evolution of intermediate mass stars with 3M⊙≤ M≤ 9M⊙ of metallicity typical of the Large Magellanic Cloud (LMC), i.e. Z = 0.008. We compare carefully the models calculated by adopting a diffusive scheme for chemical mixing, in which nuclear burning and mixing are self-consistently coupled, while the eddy velocities beyond the formal convective core boundary are treated to decay exponentially, and those calculate d with the traditional instantaneous mixing approximation. We find th at: i) the physical and chemical behaviour of the models during the H-burning phase is independent of the scheme used for the treatment of mixing inside the CNO burning core; ii) the duration of the He-burning phase relative to the MS phase is systematically longer in the diffusive models, due to a slower redistribution of helium to the core from the outer layers; iii) the fraction of time spent in the blue part of the clump, compared to the stay in the red, is larger in the diffusive models. The differences described in points ii) and iii) tend to vanish for M > 6M⊙. In terms of the theoretical interpretation of an open cluster stellar popu lation, the differences introduced by the use of a self-consistent scheme for mixing in the core with adjacent exponential decay are relevant for ages in the range 80 Myr < t < 200 Myr. These results are robust, since they are insensitive to the choice of the fr ee-parameters regulating the extension of the extra-mixing region.

Dredge‐up and Envelope Burning in Intermediate‐Mass Giants of Very Low Metallicity

The evolution of intermediate-mass stars at very low metallicity during their final thermal pulse asymptotic giant branch (AGB) phase is studied in detail. As representative examples, models with initial masses of 4 and 5 M☉ and with a metallicity of Z = 0.0001 ([Fe/H] ~ -2.3) are discussed. The one-dimensional stellar structure and evolution model includes time- and depth-dependent overshooting motivated by hydrodynamic simulations, as well as a full nuclear network and time-dependent mixing. Particular attention is given to high time and space resolution to avoid numerical artifacts related to third dredge-up and hot bottom burning predictions. The model calculations predict very efficient third dredge-up that mixes the envelope with the entire intershell layer or a large fraction thereof and in some cases penetrates into the C/O core below the He shell. In all cases primary oxygen is mixed into the envelope. The models predict efficient envelope burning during the interpulse phase. Depending on the envelope-burning temperature, oxygen is destroyed to varying degrees. The combined effect of dredge-up and envelope burning does not lead to any significant oxygen depletion in any of the cases considered in this study. The large dredge-up efficiency in our model is closely related to the particular properties of the H shell during the dredge-up phase in low-metallicity very metal-poor stars, which is followed here over many thermal pulses. During the dredge-up phase, the temperature just below the convective boundary is large enough for protons to burn vigorously when they are brought into the C-rich environment below the convection boundary by the time- and depth-dependent overshooting. H-burning luminosities of 105 to ~2 × 106 L☉ are generated. C, and to lesser degree O, is transformed into N in this dredge-up overshooting layer and enters the envelope. The global effect on the CNO abundance is similar to that of hot bottom burning. If the overshoot efficiency is larger, then dredge-up H burning causes a further increase in the dredge-up efficiency. After some thermal pulses, the dredge-up proceeds through the He shell and into the CO core beneath. Then neutrons may not be released from 13C in radiative conditions during the interpulse phase because of the scarcity of α-particles for the 13C(α,n)16O reactions. Conditions for the s-process are discussed qualitatively. The abundance evolution of H, He, C, N, O, and Na is described. Finally, the model predictions for sodium and oxygen are compared with observed abundances. The notion that massive AGB stars are the origin of the O-Na abundance anticorrelation in globular cluster giants is not consistent with the model predictions of this study. The abundance of the C-rich extremely metal-poor binaries LP 625-44, CS 29497-030, and HE 0024-2523 is discussed.

The effects of Rotation on Wolf–Rayet Stars and on the Production of Primary Nitrogen by Intermediate Mass Stars

We use the rotating stellar models described in the paper by A. Maeder &amp; G. Meynet in this volume to consider the effects of rotation on the evolution of the most massive stars into and during the Wolf–Rayet phase, and on the post-Main Sequence evolution of intermediate mass stars. The two main results of this discussion are the following. First, we show that rotating models are able to account for the observed properties of the Wolf–Rayet stellar populations at solar metallicity. Second, at low metallicities, the inclusion of stellar rotation in the calculation of chemical yields can lead to a longer time delay between the release of oxygen and nitrogen into the interstellar medium following an episode of star formation, since stars of lower masses (compared to non-rotating models) can synthesize primary N. Qualitatively, such an effect may be required to explain the relative abundances of N and O in extragalactic metal–poor environments, particularly at high redshifts.

Formation and Early Evolution of Intermediate Mass Stars

In this paper I discuss our current understanding of the formation of intermediate mass stars and the evolution of their circumstellar material. The results of the search for clusters around Herbig Ae/Be stars are discussed in the context of the possible relationship of the formation of massive stars via coalescence process. Current surveys are not able to offer a definite answer on whether there is a physical relationship between the most massive intermediate mass stars and the presence of clusters. The present day stellar densities in these clusters is too low to allow for frequent stellar collisions, but do not exclude the possibility of coalescence at earlier cluster ages. I review the evidence for and properties of circumstellar disks around Herbig Ae stars, as well as recent observational evidence for dust evolution within these disks. Evidence is presented for the presence of evolved dust in some of these disks, with grain growth to sizes as large as a few centimeters. The status of the search for the more elusive disks around Herbig Be stars is also discussed. Current observations show that disks around Herbig Be stars progenitors may have been common and massive in the protostellar phase, but optically visible Herbig Be stars, albeit very young, are never found to be surrounded by conspicuous dusty disks, suggesting dispersion timescales as short as 1 Myr.

Chemical Abundances of Newly Discovered Planetary Nebulae in the Galactic Bulge

The galactic bulge is a structure whose properties have been deeply studied recently. Its connection with other spheroidal structures as globular clusters or elliptical galaxies has been also examined under many aspects. However, many questions concerning its chemical and dynamical evolution, as well as the kinematics of their components, are still open. In particular, the study of bulge Planetary Nebulae (PNe) brings important information on the evolution of light element abundances (He,O,N,Ne,Ar,S), which are associated to the evolution of intermediate mass stars.

Hubble Space TelescopeGoddard High Resolution Spectrograph Observations of the Be + sdO BinaryφPersei

Mass transfer during the evolution of intermediate-mass stars in a close binary system can result in a rejuvenated and spun-up secondary star (which may appear as a rapidly rotating Be star) orbiting an unseen, stripped-down, remnant companion. One of the best candidates for such a system is the long- period (127 days) binary / Per. Here we present new Hubble Space Telescope Goddard High Resolution Spectrograph spectra of / Per in several UV regions that show clearly for the -rst time the spectral signature of the faint remnant companion. We derive a double-lined solution for the radial velocity curve that yields masses of 9.3 ^ 0.3 and 1.14 ^ 0.04 for the Be star and companion, respectively. A M _ M _ Doppler tomographic reconstruction of the secondary spectrum shows a rich spectrum dominated by sharp Fe IV and Fe V lines, similar to those observed in hot sdO stars. Non-LTE spectrum synthesis indicates that the subdwarf has temperature kK and gravity log g 4.2 ^ 0.1 and that the T eff 53 ^ 3 subdwarfEtoEBe star Nux ratio is 0.165 ^ 0.006 and 0.154 ^ 0.009 for the 1374 and 1647 regions, Ae respectively. The spectrum of the Be primary appears normal for a very rapidly rotating early B-type star, but we argue that the star is overluminous for its mass (perhaps owing to accretion-induced mixing). Additional sharp lines of Fe IV appear when the companion is in the foreground, and we show that these form in a heated region of the Be starIs disk that faces the hot subdwarf. Subject headings: stars: binaries: spectroscopic E stars: emission-line, Be E stars: evolution E stars: individual (/ Persei, HD 10516) E stars: subdwarfs

Evolution of intermediate mass stars

We summarize the evolution of intermediate-mass stars, calling attention to the uncertainties related to mixing in convectively unstable regions, and to recent developments in the theory of AGB stars with envelope burning. Finally, we briefly report on the distribution of C-stars of the Large Magellanic Cloud in the Mbol-log(age) plane.

Effects of envelope overshoot on intermediate mass stars

We find that a certain amount of non-local overshoot at the base of the outer convective envelope affects the evolution of intermediate mass stars in the H-R diagram, producing extended loops even for models computed with significant overshoot from the central convective core, improving in this way upon a weak point of models with core overshoot.

The H-R diagram of the Large Magellanic Cloud and implications for stellar evolution

view Abstract Citations (141) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The H-R Diagram of the Large Magellanic Cloud and Implications for Stellar Evolution Fitzpatrick, Edward L. ; Garmany, Catharine D. Abstract In this paper we produce an H-R diagram for the Large Magellanic Cloud using a sample of 1375 "normal" spectral class O through G supergiants. Our definition of "normal" includes all O stars that fit into the standard two-dimensional spectral classification framework and all later type stars that do not exhibit optical emission lines. The sample thus excludes Wolf-Rayet stars and the luminous blue variables. The upper luminosity limit observed for this sample agrees with previous estimates and indicates that only stars initially less massive than ~40 M_sun_ evolve to the red supergiant region. Between luminosities of M_bol_ = -8.5 and M_bol_ = -6.0 we find a discontinuity in the density distribution of stars in the region of the blue supergiants. On the hotter side of a line we refer to as the "ledge," the density of stars is over 5 times greater than on the cooler side. We identify this feature with an evolutionary time scale change occurring during the core helium-burning phase of the evolution of intermediate mass stars (20-30 M_sun_ > M > 10 M_sun_). We suggest that this feature is best reproduced by stellar evolution models which contain "blue loops." With this scenario, many of the observed blue supergiants are expected to be in a post-red supergiant phase of evolution. This hypothesis is consistent with suggestions that many supergiants show evidence for contamination of their surface chemical composition by the products of nuclear burning. Such contamination would occur during the red supergiant phase through the action of a surface convection zone. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169322 Bibcode: 1990ApJ...363..119F Keywords: G Stars; Hertzsprung-Russell Diagram; Magellanic Clouds; O Stars; Stellar Evolution; Supergiant Stars; Interstellar Extinction; Star Distribution; Stellar Luminosity; Stellar Models; Stellar Spectra; Astrophysics; GALAXIES: MAGELLANIC CLOUDS; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; STARS: SUPERGIANTS full text sources ADS | data products SIMBAD (4) NED (2)