scholarly journals Asteroseismic Observations of Hot Subdwarfs

2021 ◽  
Vol 8 ◽  
Author(s):  
A. E. Lynas-Gray
Keyword(s):  
Atomic Physics ◽  
Stellar Evolution ◽  
Late Stage ◽  
Interior Structure ◽  
Critical Test ◽  
Stellar Envelope ◽  
Obvious Reason ◽  
Hot Subdwarfs ◽  
Red Giant ◽  
Insight Into

There are a number of reasons for studying hot subdwarf pulsation; the most obvious being that these stars remain a poorly understood late-stage of stellar evolution and knowledge of their interior structure, which pulsation studies reveal, constrains evolution models. Of particular interest are the red giant progenitors as in looking at a hot subdwarf we are seeing a stripped-down red giant as it would have been just before the Helium Flash. Moreover, hot subdwarfs may have formed through the merger of two helium white dwarfs and their study gives insight into how such a merger may have happened. A less obvious reason for studying pulsation in hot subdwarfs is that they provide a critical test of stellar envelope opacities and the atomic physics upon which they depend.

Late-stage stellar evolution: impact on planets

2009 ◽  
Vol 8 (3) ◽  
pp. 241-242
Author(s):  
Roberto Silvotti
Keyword(s):  
Stellar Evolution ◽  
Late Stage ◽  
Short Review ◽  
The Sun ◽  
The Earth ◽  
Red Giant ◽  
Critical Phases ◽  
The Many ◽  
The Impact ◽  
Giant Branch Stars

AbstractAmong the many cosmic cataclysms that can influence life, we focus on the impact that the most critical phases of the late-stage stellar evolution may have on planets. To give an example, the red giant expansion of the Sun will probably engulf the inner planets, including perhaps the Earth. Although these studies are far from being able to predict anything concerning life, they are starting to make preliminary predictions about what can happen to planets within a few AU of the star. We present a short review of this field, in particular regarding the first detections of old evolved planets orbiting post-red giant branch stars.

scholarly journals Summary of Eighth Meeting on Hot Subdwarfs and Related Objects

2017 ◽  
Vol 26 (1) ◽  
Author(s):  
Anthony Eugene Lynas-Gray
Keyword(s):  
Stellar Evolution ◽  
Stellar Envelope ◽  
Hot Subdwarfs

AbstractSubjects discussed during the Eighth Meeting on Hot Subdwarfs and Related Objects are summarised. In particular, a succinct presentation is given of challenges to stellar evolution arising from recent observations. Mention is also made of topics not covered in the meeting such as the need for small revisions to stellar envelope opacities currently in use. I conclude with a brief discussion of current developments which will influence future hot subdwarf research and necessary initiatives that need to be taken.

scholarly journals Catalogue of the central stars of planetary nebulae

2020 ◽  
Vol 640 ◽  
pp. A10
Author(s):  
W. A. Weidmann ◽  
M. B. Mari ◽  
E. O. Schmidt ◽  
G. Gaspar ◽  
M. M. Miller Bertolami ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
Late Stage ◽  
Planetary Nebulae ◽  
Surface Gravity ◽  
Physical Parameters ◽  
Evolutionary Models ◽  
Pertinent Information ◽  
Central Stars ◽  
Statistical Improvement ◽  
Magnitude Estimates

Planetary nebulae represent a potential late stage of stellar evolution, however, their central stars (CSPNe) are relatively faint and, therefore, pertinent information is available for merely < 20% of the Galactic sample. Consequently, the literature was surveyed to construct a new catalogue of 620 CSPNe featuring important spectral classifications and information. The catalogue supersedes the existing iteration by 25% and includes physical parameters such as luminosity, surface gravity, temperature, magnitude estimates, and references for published spectra. The marked statistical improvement enabled the following pertinent conclusions to be determined: the H-rich/H-poor ratio is 2:1, there is a deficiency of CSPNe with types [WC 5-6], and nearly 80% of binary central stars belong to the H-rich group. The last finding suggests that evolutionary scenarios leading to the formation of binary central stars interfere with the conditions required for the formation of H-poor CSPN. Approximately 50% of the sample with derived values of log L⋆, log Teff, and log g, exhibit masses and ages consistent with single stellar evolutionary models. The implication is that single stars are indeed able to form planetary nebulae. Moreover, it is shown that H-poor CSPNe are formed by higher mass progenitors. The catalogue is available through the Vizier database.

scholarly journals Eclipsing binaries in the OGLE variable star catalogue: long-period evolved systems

2020 ◽  
Vol 496 (1) ◽  
pp. 550-563
Author(s):  
Barış Hoyman ◽  
Sara Bulut ◽  
Orkun Özdarcan ◽  
Ömür Çakırlı
Keyword(s):  
Stellar Evolution ◽  
Binary Stars ◽  
Eclipsing Binaries ◽  
Parameter Determination ◽  
Atmospheric Parameter ◽  
Physical Parameters ◽  
Giant Stars ◽  
Red Giant ◽  
Type Giant ◽  
Stellar Masses

ABSTRACT Red giant stars are proving to be an exceptional source of information for testing models of stellar evolution, as photometric and spectroscopic analysis has opened up a window into their interiors, providing an exciting chance to develop highly constrained stellar models. In this study, we present a determination of precise fundamental physical parameters belonging to five detached, double-lined, eclipsing binary stars in the Large and Small Magellanic Clouds containing G- or early K-type giant stars with extended envelopes. We also derived the distances to the systems by using a temperature–colour relation and compared these distances with the measurements provided in the literature. The measured stellar masses are in the range 1.8–3.0 M⊙ and comparison with the PAdova and TRieste Stellar Evolution Code (PARSEC) isochrones gives ages between 0.4 and 1.1 Gyr. The derived uncertainties for individual masses and radii of components are better than 3 and 7 per cent, respectively, for these systems. Additionally, we performed atmospheric parameter determination and [M/H] analysis for each, where we disentangled the spectra.

scholarly journals Variations of the mixing character of dipolar mixed modes in red giant stars

2020 ◽  
Vol 495 (1) ◽  
pp. 621-636 ◽  
Author(s):  
C Jiang ◽  
M Cunha ◽  
J Christensen-Dalsgaard ◽  
QS Zhang
Keyword(s):  
Asymptotic Expansion ◽  
Stellar Evolution ◽  
Coupling Strength ◽  
Stellar Model ◽  
Quality Data ◽  
Chemical Abundances ◽  
Giant Stars ◽  
Mixing Character ◽  
Mixed Modes ◽  
Red Giant

ABSTRACT Because of the high-quality data of space missions, the detection of mixed modes has become possible in numerous stars. In this work, we investigate how the mixing character of dipolar mixed modes changes with stellar evolution, as well as with frequency within each stellar model. This is achieved by monitoring the variations in the coupling strength and the period spacing of dipolar mixed modes in red-giant models. These parameters are measured by fitting the asymptotic expansion of mixed modes to the model frequencies of a grid of red-giant models with masses between 1.0 and 2.0 M⊙ and three different chemical abundances. The coupling strength and the period spacing decrease with stellar evolution. We find that the slopes of their decreasing trends depend on the radial order of the pressure mode component. A non-negligible increase of the coupling strength with frequency by up to around 40 per cent is found in the observable frequency range for a set of red-giant models. On the contrary, no significant changes of the period spacing with frequency are found. The changes in the mixing character of the modes are in most cases affected by the model mass and metallicity. Buoyancy glitches also have an impact on the mixing character. Significant fluctuations in the estimated coupling strength and period spacing are found for models approaching the luminosity bump, if the glitch impact of the frequencies is not considered in the applied asymptotic expansion.

scholarly journals Examining the nature of resource-based processes in new venture development through a business-duality lens: A farming sector taxonomy

2012 ◽  
Vol 30 (2) ◽  
pp. 161-188 ◽  
Author(s):  
Helen Perks ◽  
Dominic Medway
Keyword(s):  
Late Stage ◽  
New Ventures ◽  
Multiple Case Study ◽  
New Venture ◽  
Entrepreneurial Process ◽  
Case Study Methodology ◽  
Multiple Case ◽  
Study Methodology ◽  
Insight Into

This article investigates the nature of resource-based processes in the development of new ventures, adopting a business duality lens. Business duality occurs where a new venture is developed alongside an established business. The research employs a multiple case study methodology situated in the farming sector. The details of resource assembly and deployment are examined and presented through four stages of the entrepreneurial process: initiation, experimentation, mature and late stage. The findings offer insight into the manner in which resource ties between the businesses relate to processes of resource assembly and deployment and in addition, inform a business duality-based taxonomy. This depicting three generic approaches to managing resource-based processes in the development of new ventures in the farming sector: holistic innovators, reactive innovators and cautious innovators. We conclude by considering the implications of our arguments for new venture activity in other business duality contexts.

scholarly journals Observations of Lithium in red giant stars

2009 ◽  
Vol 5 (S268) ◽  
pp. 301-309
Author(s):  
Verne V. Smith
Keyword(s):  
Stellar Evolution ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Asymptotic Giant Branch Stars ◽  
Multiple Sources ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Red Giant ◽  
First Time ◽  
Giant Branch Stars

AbstractConnections between observations of the lithium abundance in various types of red giants and stellar evolution are discussed here. The emphasis is on three main topics; 1) the depletion of Li as stars ascend the red giant branch for the first time, 2) the synthesis of 7Li in luminous and massive asymptotic giant branch stars via the mechanism of hot-bottom burning, and 3) the possible multiple sources of excess Li abundances found in a tiny fraction of various types of G and K giants.

scholarly journals Presentation and Interpretation of High Resolution Infrared Spectra of Late-Type Stars

1974 ◽  
Vol 3 ◽  
pp. 255-268 ◽  
Author(s):  
R. I. Thompson
Keyword(s):  
Stellar Evolution ◽  
Carbon Star ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Spectra ◽  
Cool Star ◽  
Convective Mixing ◽  
Processed Material ◽  
Carbon Compounds ◽  
Red Giant

Current interest in stellar evolution is concentrated on the life of a star after it has left the main sequence. Of particular interest are the red giant or supergiant periods during the hydrogen and helium shell burning phases. Convective mixing during these stages can mix nuclear processed material to the surface where it may be viewed by spectroscopic methods. It is imperative that this rare chance to view processed material be exploited fully to increase our knowledge of stellar evolution.The observation and interpretation of cool star spectra has its own particular set of problems and advantages. A particular difficulty is the formation of molecules at the low temperatures which occur in the atmospheres of late stars. Not only must the particularly complex spectra of molecules be dealt with but the problem of chemical equilibrium in the atmosphere must be solved accurately before quantitative analysis may be performed. The formation of molecules, however, has one advantage in that it very dramatically separates those stars with carbon to oxygen ratios greater than one from those with ratios less than one. It is the very high dissociation energy of 11.1 eV for the CO molecule which performs this separation. If carbon is less abundant than oxygen all of the carbon is tied up in CO and only oxides are formed in the stellar atmosphere which produce typical M star spectra. If, however, carbon is more abundant than oxygen then carbon compounds such as C2 are formed in place of the oxides and a carbon star spectrum is formed. One of the great advantages of infrared stellar spectra is that it is the only ground based technique for observing CO in stellar atmospheres.

scholarly journals Giant Planets, Tiny Stars: Producing Short-period Planets around White Dwarfs with the Eccentric Kozai–Lidov Mechanism

2021 ◽  
Vol 922 (1) ◽  
pp. 4
Author(s):  
Alexander P. Stephan ◽  
Smadar Naoz ◽  
B. Scott Gaudi
Keyword(s):  
Direct Evidence ◽  
Giant Planet ◽  
Giant Planets ◽  
Tidal Effects ◽  
Stellar Envelope ◽  
Orbital Parameters ◽  
Short Period ◽  
Migration Mechanism ◽  
Red Giant ◽  
Host Stars

Abstract The recent discoveries of WD J091405.30+191412.25 (WD J0914 hereafter), a white dwarf (WD) likely accreting material from an ice-giant planet, and WD 1856+534 b (WD 1856 b hereafter), a Jupiter-sized planet transiting a WD, are the first direct evidence of giant planets orbiting WDs. However, for both systems, the observations indicate that the planets’ current orbital distances would have put them inside the stellar envelope during the red-giant phase, implying that the planets must have migrated to their current orbits after their host stars became WDs. Furthermore, WD J0914 is a very hot WD with a short cooling time that indicates a fast migration mechanism. Here, we demonstrate that the Eccentric Kozai–Lidov Mechanism, combined with stellar evolution and tidal effects, can naturally produce the observed orbital configurations, assuming that the WDs have distant stellar companions. Indeed, WD 1856 is part of a stellar triple system, being a distant companion to a stellar binary. We provide constraints for the orbital and physical characteristics for the potential stellar companion of WD J0914 and determine the initial orbital parameters of the WD 1856 system.

scholarly journals A CHEOPS white dwarf transit search

2021 ◽  
Vol 651 ◽  
pp. L12
Author(s):  
Brett M. Morris ◽  
Kevin Heng ◽  
Alexis Brandeker ◽  
Andrew Swan ◽  
Monika Lendl
Keyword(s):  
Stellar Evolution ◽  
Point Spread Function ◽  
White Dwarf ◽  
Metal Pollution ◽  
White Dwarfs ◽  
Point Spread ◽  
Spread Function ◽  
Data Products ◽  
Red Giant

White dwarf spectroscopy shows that nearly half of white dwarf atmospheres contain metals that must have been accreted from planetary material that survived the red giant phases of stellar evolution. We can use metal pollution in white dwarf atmospheres as flags, signalling recent accretion, in order to prioritize an efficient sample of white dwarfs to search for transiting material. We present a search for planetesimals orbiting six nearby white dwarfs with the CHaracterising ExOPlanet Satellite (CHEOPS). The targets are relatively faint for CHEOPS, 11 mag < G < 12.8 mag. We used aperture photometry data products from the CHEOPS mission as well as custom point-spread function photometry to search for periodic variations in flux due to transiting planetesimals. We detect no significant variations in flux that cannot be attributed to spacecraft systematics, despite reaching a photometric precision of < 2 ppt in 60 s exposures on each target. We simulate observations to show that the small survey is sensitive primarily to Moon-sized transiting objects with periods between 3 h < P < 10 h, with radii of R ≳ 1000 km.

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