90 research outputs found
Exploring the evolution of classical cepheids with MESA
No full textClassical Cepheid stars play an important role in the rst step of the distance ladder
determination in the Milky Way and in nearby galaxies. The period-luminosity rela-
tionship of classical Cepheid stars is used to calculate the Hubble constant which is
a measurement of the expansion of the universe. Using Modules for Experiments in
Stellar Astrophysics (MESA), new evolution models with and without convective core
overshooting are used to probe for the period rate change at the di erent crossing of
the instability strip. A population synthesis analysis shows that 63.4% of classical
Cepheids have positive period rate change while the rest have negative period rate
change. With overshooting, 91.1% of classical Cepheid stars have positive period rate
change. To account for the di erence in the number of classical Cepheid stars that are
observed versus those that are modeled with MESA, mass-loss, rotation and metallic-
ity need to be added to the models to help explain the di erence with observations
Manufacturer Company's Inventory Accuracy Affecting Factors based on Hilding Anders Baltic PLC
No full textEnamus ettevõtteid, kuid kindlasti kõik tootmisettevõtted soovivad saavutada kõrget varude täpsust. Ka madratsite ja madratsikatetele tootmisele spetsialiseerunud ettevõttes Hilding Anders Baltic AS peetakse oluliseks varude täpsust. Varude täpsus aitab tagada tootmisettevõttes sujuva tootmisprotsessi, kus ei toimu seisakuid ootamatute varude puudujäägi tõttu. Samuti aitab varude täpsus tagada sujuva kaupade väljastamise protsessi, kus ei teki olukorda, et toodet ei ole hoiukohal, kuid arvestuslikult tarkvarasüsteemis näitab, ja toode jääb selletõttu kliendile õigel ajal tarnimata. Kiirus ja kokkulepitud tähtaegadest kinnipidamine on aga iga ettevõtte üheks oluliseks konkurentsieeliseks. Käesoleva lõputöö autor keskendus varude täpsust mõjutavate tegurite järgimissüsteemi parandamisele ettevõtte Hilding Anders Baltic AS näitel. Ehkki töö põhineb Hilding Anders Baltic AS andmetel, võib varude täpsust mõjutavate tegurite järgimise süsteemi kasutada ka teistes tootmisega tegelevates ettevõtetes. Antud lõputöö autorit ajendas keskenduma varude täpsust mõjutavate tegurite järgimissüsteemi parandamisele asjaolu, et Hilding Anders Baltic AS inventuurid on näidanud, et laovarude kogustel tarkvarasüsteemis ja tegelikel kogustel laos on märkimisväärsed erinevused. Nagu ka David P. Piasecki oma soovitustes kirjutas, tuleks enne uute tehnoloogiliste lahenduste kasutusele võttu vaadata üle kuidas tema poolt kirjapandud põhitõdesid ettevõttes järgitakse. Muutes vajadusel olukorda tema ettepanekutele toetudes, lubas Piasecki varude täpsuse paranemist. Käesoleva lõputöö teooria osas tõi autor välja milliseid probleeme ja millest tingituna võib tootmisettevõttes ette tulla. Kaheks peamiseks põhjuseks peetakse töötajate teadmiste puudumist (tehtava töö protseduuridest) ja vähest keskendumisvõimet. Teooria osa kõige olulisemaks peatükiks oli „Põhitõed saavutamaks parem varude täpsus“, kus oli välja toodud eelmises lõigus mainitud autori David P. Piasecki põhitõed. Täpsemalt 11 etappi ja nende etappide sisuline osa. Veel oli teooria osas tutvustatud ettevõtet AS Hilding Anders Baltic ja käesoleva lõputööga seonduvalt ka ettevõtte tootmist, ladu ja materjalide ning toodete liikumist. Antud lõputöö teises osas tõi autor välja uurimuse metoodika ja kirjeldas uurimuse etapid. Täpsemalt oli kirjeldatud, kuidas autor viis läbi tema poolt väljavalitud andmekogumismeetodid, mida need endast kujutasid, mis olid nende eelised ja puudused. Andmekogumismeetoditeks oli statistiline andmeanalüüs, avatud ja varjatud vaatlused ning struktureeritud intervjuu. Autor seletas lahti ka kuidas ja mille põhjal nimetatud uurimusmeetodid läbi viis ja kuidas moodustus valim. Käesoleva lõputöö kolmas osa sisaldas uurimusandmete analüüsi ja sünteesi. Töö esimesteks algandmeteks olid 2014. aasta aprillis ja oktoobris toimunud inventuuride tulemused, millede andmetele toetudes leidis autor ettevõttes kõige problemaatilisema ala tooraine tsoonis, tootmise tsoonis ja valmistoodangu tsoonis. Kõige problemaatilisemateks aladeks kujunesid alad, kus erinevused füüsiliselt laos ja arvestuslikult tarkvarasüsteemis olevatel kogustel olid rahalises väärtuses kõige suuremad. Nendel aladel viis autor läbi avatud ja varjatud vaatlused, kus vaatles ametikohti, mis puutuvad kokku varude koguste muutmisega füüsiliselt või arvestuslikult tarkvarasüsteemis. Kuigi vaatlused olid läbi viidud ainult kõige problemaatilisematel aladel, aitavad saadud uurimuse tulemused ja järeldused parandada varude täpsust ka tehase teistel aladel. Vaatlustabelite algandmeteks, ehk teisteks algandmeteks olid ettevõtte protseduuride kirjeldused - standardoperatsioonide protseduurid ja tööjuhendid, mille järgi peavad töötajad tööd tegema. Nendest filtreeris autor vaatlustabelite jaoks välja info, mis oli seotud varude koguseliste muutmistega tarkvarasüsteemis või füüsiliselt. Kolmandaks algallikaks oli juba mainitud David P. Piasecki poolt kirjapandud põhitõed, millede andmetele toetudes koostas autor struktureeritud intervjuud, mille viis läbi tootmis- ja logistikajuhiga. Autor viis need läbi eesmärgiga saada teada, kuidas järgitakse ettevõttes varude täpsust mõjutavaid tegureid. Uurimuse käigus leitud andmete analüüsimise ja tõlgendamise järel tegi autor tulemustest järeldused ja ettepanekud. Töö empiirilises osas läbi viidud analüüs näitas, et varude täpsust mõjutavate tegurite järgimissüsteem ettevõttes Hilding Anders Baltic AS vajab parandamist. Inventuuride tulemused olid selle esimeseks tõestuseks. Vaadates vaatlustabeleid, milledes oli rõhutatud erinevused kirjapandud protseduuride ja autori poolt läbi viidud varjatud ning avatud vaatluste käigus selgunu vahel, oli näha, et erinevusi oli palju. Tootmisjuhi ja logistikajuhiga läbi viidud struktureeritud intervjuude eesmärk oli saada ülevaadet selle kohta, kuidas ettevõttes Hilding Anders Baltic AS järgitakse 11 põhitõde. Olukorda aitasid mõista ka ettevõtte spetsialistidega läbi viidud avatud intervjuud, mis andsid autorile lisainfot ka spetsialisti vaatenurgast. Kõikide intervjuude tulemused koos vaatlustega andsid autorile ülevaate, kuidas ettevõttes Hilding Anders Baltic AS järgitakse 11 põhitõde ja autor tegi nendest järeldused ning ettepanekud olukorra parandamiseks. Autori peamised ettepanekud olid seotud kuue põhietapiga. Nendeks olid vaadata üle ja ajakohastada protseduuride kirjeldused, kontrollida regulaarselt töötajate teadmisi protseduuride täitmisest, jälgida kuidas protseduure täidetakse ja kuidas need toimivad, koolitada laotöötajaid tööjuhendit kasutades, alustada tsükliliste inventuuridega ja panna paika protsesside ja protseduuride regulaarse ümberhindamise süsteem. Ettevõte Hilding Anders Baltic AS saab oluliselt parandada oma varude täpsust kui viib sisse autori poolt pakutud ettepanekud. Lisaks sellele tuleb aluseks võtta ka korduvalt nimetatud 11 põhitõde, millele toetudes varude täpsust mõjutavate tegurite järgimise süsteemi saab parandada.Manufacturer Company's Inventory Accuracy Affecting Factors based on Hilding Anders Baltic PLC. The company Hilding Anders Baltic PLC is engaged in the manufacturing of mattresses and mattress covers in Estonia. For the smooth dispense of production it is important that materials and products inventories are accurate. Two stocktaking results showed that inventories in PLC Hilding Ander Baltic are inaccurate. In general, when it comes to improving the accuracy of the inventory, then people usually start from technology. People explore what resources need to be replaced or purchased. In this thesis the author used a different approach, since the company does not want to make any additional investments to remedy the situation, and happy to find cheaper solutions to improve the situation. The author of the study designates that the problem of this study is that the factors affecting and improving the accuracy of the inventory are not followed properly in the Hilding Anders Baltic PLC. There are many factors that affect the accuracy of the inventory and the author points out the number of steps that by the following can help to reduce inventory difference. The thesis aims to improve the observance system of the factors affecting and improving the accuracy of the inventory in the enterprise. Author sets such a goal because additionally to the conventional drawbacks due to human factors, errors may also result of insufficient implementation of other activities related to the inventory accuracy. First initial data of the thesis work were results of inventories that took place in April and October in 2014. Based on these data the author found three most problematical areas - one in raw-material zone, one in production zone and one finished goods zone. The most problematical areas are areas where the difference (in monetary value) between physical quantities on the storage space and quantities in software system is biggest. In these areas author observed workers who can change the quantity of inventory physically or in the software system. Though observations were held only in the most problematical areas the results and findings can improve inventory accuracy also in other areas of the company. Initial data in the observation tables, i.e. the second initial data, were the company's descriptions of procedures - standard operational procedures and work instructions, according to which workers must work. Of those author filtered the information for the observation tables which is related to the quantitative stock changes in the system software or physically. Third data source was David P. Piasecki and his main stages. The data of those basics was used by the author on the structured interview, which was conducted by the Head of Production and Head of Logistics. Interviews were conducted to find out how the company follows the factors affecting the accuracy of the inventory. After the data was analyzed the author made conclusions and proposals. The analysis carried out in the empirical part of the thesis showed that the compliance system of factors affecting the accuracy of inventory the company Hilding Anders Baltic PLC needs to be improved. The results of the inventories are the first proof of that. Then, looking the observation tables, where the differences between company`s written procedure rules and information revealed during concealed and open surveys conducted by the author are highlighted in bold letters, we can see that there are many differences. The objective of the structured interviews conducted with the Head of Production and Head of Logistics was to obtain an overview of how the company Hilding Anders Baltic AS follow the 11 main stages. To get better understanding of the situation author had also interviews with several specialists in the company, who gave information from their point of view. The results of all interviews and observations gave overview about how are followed 11 main stages in PLC Hilding Anders Baltic. Author made her conclusions and suggestions how to improve the situation. Main suggestions were connected with six main stages. First need to review and update the procedure documentations and then begin to test employee`s knowledge about documented procedures. It is important to start observing how procedures are carried out and through this get information that needs to be improved in documents. Warehouse workers should be trained using working instructions. Fifth suggestion is to start with cycle counting that gives better overview of inventory. Sixth suggestion that author made, was to revaluate regularly processes and procedures. The author of present thesis focused on improving the observance system of the factors affecting and improving the accuracy of the inventory in the enterprise. Though the thesis bases on the data of Hilding Anders Baltic PLC, it is possible to use observance system of the inventory accuracy affecting factors successfully in other production companies. If PLC Hilding Anders Baltic implements all the suggestions made by the author then it is possible to achieve better inventory accuracy
Production Line Process Optimization Through Standardization On The Example Of Hilding Anders Baltic PLC Company
No full textHilding Anders Baltic AS nagu teisedki tootmisettevõtted on huvitatud tootmisprotsesside efektiivsuse suurendamises. See aitab vähendada kulusid lisatööjõu värbamisele ja maksimaalselt kasutada ettevõtte ressursse. 2015. aasta lõpuks ettevõte Hilding Anders Baltic AS suurendas oma tootmismahtu ja tootesortimenti. Tootesortimendi suurenemisel tekkis kettalõikuse tootmiskeskuses lisatöötaja rakendamise vajadus ning kangajääkide suurenemine. Edasi autor keskendus protsesside optimeerimise võimalustele ja nende rakendamisele Hilding Anders Baltic AS ettevõtte näitel. Teoreetilises osas autor tõi välja metodoloogia, mille abil on võimalik optimeerida protsessi. Aluseks oli võetud Lean Six Sigma, mis on kombineeritud Lean ja Six Sigma erinevatest metodoloogiatest. Lean keskendub kaheksa raiskamise vähendamisele, aga Six Sigma – variatsiooni vähendamisele. Ettevõttes autor viis läbi projektipõhilise töö, mis koosnes viiest etapist. Need etapid kuuluvad DMAIC-metodoloogiasse, mis on üheks probleemide lähenemisviisiks, ja kujutavad endast probleemi defineerimist, andmete kogumist ehk mõõtmist, kogutud andmete analüüsimist, parendusettepanekute väljatöötamist ja ellu viimist ning viimaseks tulemuste ohjamist. Lisaks tegi autor tutvustust ettevõttega Hilding Anders Baltic AS ja tegi ammendava ülevaade madratsite ja madratsikatete tootmisest ja logistilistest protsessidest. Käesoleva lõputöö metoodika osas on autori poolt täpsemalt kirjutatud uurimistöö käik. Autor valis projektipõhise töö tegemise antud ettevõttes ja selleks oli valitud tegevusuurimus. Vastavalt DMAIC-metodoloogiale olid kirjeldatud iga etapi kasutatud meetodid: huvigrupi kaart, SIPOC, ajurünnakud, riskide lühianalüüs, parendamise kavand, andmete kogumise plaan, väärtusvoo kaardistus ja analüüs, vaatlus, poolstruktureeritud intervjuud, raiskamiste väljaselgitamine, parendusettepanekute kokkuvõte, tulemuste kontroll ja ohjamine auditi ja mõõdiku rakendamise abil. Autor seletas lahti ka mida need meetodid endast kujutavad. Empiirilises osas autor selgitas välja et, lisatööjõu kasutamise vajaduse ja kanga jääkide suurenemine oli seotud toodete tüüpide variatsiooni kasvuga: tehases käsitletav kangaste artiklite arv oli 01.02.2015 seisuga 103, aasta pärast samal ajal oli ta 190. Kettalõikuse töökeskuse artiklite variatsioon suurenes vastavalt 1,8 korda (190÷103). Probleemide lahendamiseks autor kutsus kokku meeskonna ja tegi liikmete analüüsi huvitatuse ja mõju taseme järgi. Lisaks oli tehtud lühike riskide analüüs ja määratud optimeerimise projekti piirid, et tegutsemissuund oleks selge. Teisel etapil autor koos meeskonnaga arutas, mis on võimalikud probleemide juurpõhjused ja mis andmeid on vaja koguda edaspidiseks analüüsimiseks. Selle põhjal oli koostatud andmete kogumise plaan, teostatud mõõtmised ja mõõtmiste tulemused võimalusel esitatud visuaalselt. Oli koostatud tegeliku protsessi väärtusvoog, mis lubas selgitada välja selle puuduseid. Jälgides kettalõikuse töökeskuse operaatori tööpäeva autor teostas põhjalikku ülevaadet toimuvast ja tuvastas mitu ajaraiskamise momenti. Väljaselgitatud probleemide põhjal autor koos meeskonnaga pakkus erinevaid lahendusi ja tegi nende analüüsi ning maksumuse ja teostamisvõimaluse hindamise. Parendusettepanekud puudutasid käsitsi plaani töötlemise muutmist poolautomatiseeritud viisi peale standardiseerimise abil, põrandapinna ja riiulite markeerimist kanga jääkide loogiliseks ja korralikumaks hoiustamiseks, abitooli ja metallkapi tööriistade jaoks kasutuselevõtt. Tehtud parendusettepanekute põhjal autoril oli võimalus teha piloot-projekt. Selle raames olid tehtud tehnilised, ergonoomilised ja protsessi muudatused. Saavutatud tulemus erines oodatud rahalisest kasust, kuid oli ikkagi positiivne. Veebruaris tööd hakati tegema ~1,44 korda kiiremini, kui see oli jaanuaris. Samas veebruari keskmine 7,3t tulemus annab võimalust teha tööd ainult ühe töötajaga mitte kasutades lisatööjõudu. Lõputöö käigus autor tutvus protsesside optimeerimise võimalustega ning rakendas need Hilding Anders Baltic AS ettevõttes ühe töökeskuse näitel. Selle jooksul oli saavutatud reaalne tulemus ja rahavõit ettevõttele. Teised tootmisettevõtted saavad ka kasutada käesolevas töös rakendatud metodoloogiaid enda tootmisprotsesside optimeerimiseks.Hilding Anders Baltic PLC like other production companies is interested in increasing the efficiency of production processes. This helps to reduce costs of additional labor recruitment and to use company’s resources by maximum. By the end of 2015, the Hilding Anders Baltic PLC company increased its production and product assortment. Increase in types of products caused a need for additional staff and increase in fabric residues at the disc cutting work center. Next, the author focused on the opportunities of processes optimization and on their implementation at Hilding Anders Baltic PLC company. In this thesis the author has set an objective to find a solution to optimize the production line process. In order to achieve the objective author chose Hilding Anders Baltic PLC disc cutting production work center as object of the study. The reason for this was the fact that the latest inventory results were negative and with a growing trend. Lean Six Sigma methodology was taken for a base, which is a combination of the two different methodologies Lean and Six Sigma. Lean focuses on eight waste reductions, but Six Sigma focuses on variance reduction. The author carried out a project at the company. It consisted of five stages. The stages are a part of DMAIC-methodology, which is one of the problem solving approaches, and are a problem definition, data collection and measurement, analysis of the made measurements, development and implementation of the suggested improvements, and, finally, control of the results. In addition, the author introduced the Hilding Anders Baltic PLC company and gave an exhaustive overview of mattresses and mattress covers production and logistical processes. The research steps were described more detailed in the methodological part of this thesis work. There was made an action research through a project in the company. According to the DMAIC-methodology there were described methods used in each step: a project charter, a stakeholder map, a SIPOC, a brainstorming, a brief analysis of risks, a data collection plan, a value stream mapping and analysis, an observation, semi-structured interviews, wastes’ identification, a summary of suggested improvements and a control of the results through an audit and key performance indicator implementation. The author has also explained what the methods they are in this part. In the empirical part the author clarified that the additional manpower need and fabric residue increase were related to increase of variation in the types of manufacturing products: the number of articles the factory dealt with 1st February 2015 accounted 103, year later at the same time it was already 190. Variation of the articles at the disc cutting work center increased relatively by 1,8 times (190÷103). In order to solve problems the author called together a team and made analysis of the level of interest and the impact on the project success of the team members. There was also made a short risks analysis and determined the start and the end of the process under scrutiny in order to have the same understanding of the further direction. In the second stage, the author discussed with the team what are the potential root causes of the problems and what data is needed to be collected for the further analysis. Next, there was composed data collection plan, made measurements and the results presented visually if it was possible. There was also mapped value stream of the actual process, which allowed to identify its weaknesses. Later, by monitoring disc cutting work center operator’s working day the author has carried out a comprehensive review of performed actions and identified several wastes of time. The author in collaboration with the team offered variety of solutions for the identified problems and did solutions’ analysis and evaluation of their costs and realization possibility. Improvement suggestions were related to the plan processing change from manual to semi-automated way through the operations’ descriptions standardizing, floor space and shelf marking for logical and more correct fabric residues storage, a supporting chair and a metal locker for tools deployment. According to the offered improvement suggestions the author had opportunity to start pilot project. There were made technical, ergonomic and process changes within the pilot project framework. Achieved result differs from the expected financial benefit, but was still positive. In February, the job was done ~1,44 times faster, than it was in January. In addition to this, average spent time per day in February was 7,3h, that gives opportunity to do the same job without using an additional manpower. To achieve better results the author advices the company to use improvement suggestions that have not been implemented during the pilot project. Within the thesis the author met with process optimization opportunities and implemented them on the example of one of the Hilding Anders Baltic PLC production lines. There were achieved actual outcome and financial benefit in the end. Other manufacturing companies can also use the methodologies implemented in the work to optimize their production processes
The Dynamic Atmospheres of Classical Cepheids: Studies of Atmospheric Extension, Mass Loss, and Shocks
No full textIn this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed.
In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.Ph
The Dynamic Atmospheres of Classical Cepheids: Studies of Atmospheric Extension, Mass Loss, and Shocks
No full textIn this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed.
In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.Ph
The Drake Equation and the Search for Life in the Galaxy through Indigenist Lenses
No full text<p>Abstract:</p>
<p>So much of the modern search for life in beyond our Solar System is motivated by the interpretation of the Drake Equation. However, much of this discourse has arisen with colonial and Eurocentric biases that limit the interpretation of the various terms of the equation. In this talk, I will present an Indigenist view of the Drake Equation and show that this perspective leads to a significantly different view of life in the Galaxy. I will also show that this perspective suggests that the current binary discussion of bio-signatures and techno-signatures in the search for intelligent life can be very limiting.</p>
Period Change and Stellar Evolution of β Cephei Stars
No full textThe β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change
Convection, Granulation, and Period Jitter in Classical Cepheids.
No full textAnalyses of recent observations of the sole classical Cepheid in the Kepler field, V1154 Cygni, found random changes of about 30 min in the pulsation period. These period changes challenge standard theories of pulsation and evolution because the period change is non-secular, and explaining this period jitter is necessary for understanding stellar evolution and the role of Cepheids as precise standard candles. We suggest that convection and convective hot spots can explain the observed period jitter. Convective hot spots alter the timing of flux maximum and minimum in the Cepheid light curve, hence change the measured pulsation period. We present a model of random hot spots that generate a localized flux excess that perturbs the Cepheid light curve and consequently the pulsation period, which is consistent with the observed jitter. This result demonstrates how important understanding convection is for modeling Cepheid stellar structure and evolution, how convection determines the red edge of the instability strip, and just how sensitive Cepheid light curves are to atmospheric physics
Cepheid in the Eclipsing Binary System OGLE-LMC-CEP1812 is a Stellar Merger
No full textClassical Cepheids and eclipsing binary systems are powerful probes for measuring stellar fundamental parameters and constraining stellar astrophysics. A Cepheid in an eclipsing binary system is even more powerful, constraining stellar physics, the distance scale and the Cepheid mass discrepancy. However, these systems are rare, only three have been discovered. One of these, OGLE-LMC-CEP1812, presents a new mystery: where the Cepheid component appears to be younger than its red giant companion. In this work, we present stellar evolution models and show that the Cepheid is actually product of a stellar merger during main sequence evolution that causes the Cepheid to be a rejuvenated star. This result raises new questions into the evolution of Cepheids and their connections to smaller-mass anomalous Cepheids
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