Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://star.arm.ac.uk/jobs/PDRA-2015/Specification-PDRA.pdf Äàòà èçìåíåíèÿ: Tue Feb 10 20:17:07 2015 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 05:07:26 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: binary star

THE GOVERNORS OF ARMAGH OBSERVATORY AND PLANETARIUM Job Description Job Title: Postdoctoral Research Assistant

This is a fixed-term position funded by a consolidated research grant to the Armagh Observatory from the Science and Technology Facilities Council. Overv iew of the Post: The Postdoctoral Research Assistant will be responsible for pursuing research on the evolution, structure and fate of double white dwarf stars which merge within a Hubble time, including the post-merger evolution of binaries which fail to explode as Type Ia supernovae. The object will be to compare the properties of postmerger models with observations of various evolved stars including hot subdwarfs. This research contributes to a wider programme: "The exotic star zoo: commonenvelope evolution, white-dwarf mergers and pulsating helium stars", for which a full description is attached. Salary and Holidays: · Based on the University Research Staff Salaries: Grade Ia, the starting salary will be in the range ¸27,274 - ¸29,784 depending on experience. · No additional payment will be made for hours worked in addition to the normal 35 hour week for the post (based around the Armagh Observatory normal office hours - 9am to 5pm). The Employer may require the person appointed to work additional hours to fully discharge the responsibilities of the post. Time off in lieu will be granted for additional hours worked. · The person appointed will be entitled to 30 days annual leave plus 11 public and privilege holidays. The leave year commences on 1 January and new entrants will be entitled to leave proportionate to completed months of service. Location: · The Observatory is located on College Hill, Armagh. Responsible To: · Professor Simon Jeffery Starting Date: · Not later than 1 October, 2015 Probation Period: · The successful candidate will be required to complete a standard period of probation, lasting 6 months.

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Key Tasks and Responsibilities Research · Carry out research on the structure and evolution of post-merger double white dwarfs and related objects. · Write up research results as papers for publication in major refereed journals. · Attend and present relevant research at appropriate workshops and conferences as the budget allows. Miscellaneous Duties · Play a full part participation in · Carry out other and capabilities the Director. in the academic life of the Observatory, including discussion meetings and seminars. duties not specified but within the general scope of the post of the post holder, as determined by the Line Manager or

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Person Specification JOB TITLE: Postdoctoral Research Assistant

Education/Qualification Essential · A good (II1 or better) first degree in physical and/or mathematical sciences. · A PhD or equivalent degree in astronomy or astrophysics. Experience Essential · At least 2 years' experience of using a major stellar evolution package (e.g. MESA, STARS, ...) · A minimum of 2 refereed papers in major journals (e.g. MNRAS, A&A, ApJ, ...). Papers in press are acceptable. Desirable · At least 1 year's experience of developing new code for astrophysical modelling · At least 1 year's experience modelling at least one of: binary star evolution, stellar pulsation, chemical diffusion. Desirable · A thesis in stellar structure and evolution, or in stellar hydrodynamics

Skills, Abilities and Knowledge Essential · Excellent understanding of stellar physics · Fluency in one or more high-level programming languages (e.g. Fortran, Python, etc.) · Competence in one or more graphical and text-processing languages (e.g. LaTeX, IDL, ...) · Good written and oral communication skills · Self-motivated and able to work without supervision Desirable · Familiarity with version-control software (e.g. git) and package building tools (e.g. automake ) · Familiarity with multi-processor computing

Shortlisting Criteria In addition to the above qualifications and experience, representatives of the Governors of the Armagh Observatory and Planetarium reserve the right to shortlist only those candidates who satisfy one or more of the desirable criteria.

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THE GOVERNORS OF ARMAGH OBSERVATORY AND PLANETARIUM Additional Information Postdoctoral Research Assistant Enquiries: Enquires about the position can be directed to Prof Simon Jeffery: csj@arm.ac.uk . Applications: Applicants should submit a Curriculum Vitae (CV), statement of research interests, and the names of three referees to: Human Resources, Armagh Observatory, College Hill, Armagh BT61 9DG, UK (hr@arm.ac.uk), to arrive no later than 3pm on 13 March 2015*. It is the candidate's responsibility to ensure that his or her application form is submitted and received by this closing date and time. No late application forms will be considered. No applications, or supporting information in respect of an already submitted application, will be considered after this date and time. Short-listing and Interv iew Date*: It is envisaged that short listing for this post will take place on 18 March 2015. If necessary, interviews will take place on 30 March - 2 April 2015.

Pre-Employment Checks: All offers of employment will be conditional, with the offer clearly being subject to the candidate's relevant pre-employment checks being returned satisfactorily. · Proof of identity and entitlement to work in Northern Ireland; · Proof of relevant qualifications; · Two references; · Criminal Convictions Disclosure. *Closing deadline extended from 2 March 2015, other key dates extended.

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ARMAGH OBSERVATORY AND PLANETARIUM - Background Information The Armagh Observatory and the Armagh Planetarium are distinct institutions within a single statutory corporation, "The Governors of the Armagh Observatory and Planetarium". The principal function of the Armagh Observatory, founded in 1789, is to undertake original research of a world-class academic standard that broadens and expands our understanding of astronomy and related sciences and to communicate this knowledge to the wider public. The corporation's incorporating legislation is the Armagh Observatory and Planetarium (Northern Ireland) Order 1995. The organisation is classified as a NonDepartmental Public Body within the Northern Ireland public sector and is sponsored by the Department of Culture, Arts and Leisure (DCAL). The governing structure comprises the Board of Governors and the Management Committee, a sub-Committee of the Board of Governors. The organisation currently receives annual funding from DCAL of approximately ¸1.5m and makes bids to DCAL for additional in-year recurrent and capital funding. The Armagh Observatory also currently receives in excess of ¸300k per annum grant funding from research funding bodies in the UK and Ireland for astronomical research projects. The Armagh Observatory is located on a 14 acre site within the City of Armagh and is led by a Director. The Observatory has a complement of approximately 30 research staff, students and support staff and regularly hosts academic guests from other institutions. The research staff make observations of stars, galaxies, the Sun, comets and asteroids etc. using both ground based telescopes in locations such as South Africa, Chile and the Canary Islands and also satellite based instruments managed by the European and Japanese Space Agencies and NASA. A number of observational programmes are located on the Observatory site including daily meteorological readings contributing to the Observatory's unique meteorological series and climate archive. The data obtained from these observational programmes are analysed using the computer facilities at Armagh Observatory and the results are published in international scientific journals. The goal is to contribute to the understanding of the universe and to communicate that knowledge to the public. Computer facilities Computer facilities currently comprise approximately 40 desktop systems (Linux and MacOS X), a central Linux server (Mail, Web, DNS, NIS, DHCP) and two ancillary web servers. Additionally two other web servers are hosted for partner organisations. The local area network extends across 6 buildings. It is CAT5 and fibre Ethernet based running at 1 Gbps with currently 11 managed switches. There is also site-wide wireless access. The site has a 100Mbps upstream and downstream internet link. The firewall is configured and managed in-house. The servers, firewall and over 100TB of RAID storage are housed in 3 dedicated computer rooms, one of which is fully air-conditioned. The RAID storage is part of
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an automated backup system and is also used for astronomical data storage. Additionally, 4 rack mounted multi-processor servers are dedicated to astronomical computations. The Observatory also has access to super-computer facilities at the Irish Centre for High-End Computing (ICHEC). Other core technical facilities include 12 networked printers, digital projectors, video-conferencing equipment, digital displays, digital still and video cameras, and network security cameras. The Observatory operates an in-house VOIP telephony system based on an Asterisk PBX with both VOIP trunk lines and ISDN lines. There are a number of active on-site monitoring and observational programmes using telescopes, CCD imaging systems and computer control systems and enclosures, for which systems support is required. Further details about the work and future plans and objectives of the Armagh Observatory, the latest set of audited accounts, minutes of the meetings of the Board of Governors and Management Committee and other information are available on the website at www.arm.ac.uk

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Research Programme: The exotic star zoo: common-envelope evolution, whitedwarf mergers and pulsating helium stars Background. Modern stellar astronomy regularly continues to discover new types of stars, frequently with properties that challenge the contemporary theory of structure and evolution. Many of these are evolved stars with low mass and hydrogen-deficient or otherwise chemically-peculiar surfaces, and include RCrB variables (RCB), extreme helium stars (EHe) and various classifications of heliumrich hot subdwarf (He-sd) (see Fig. 1). These stars are important for understanding the diversity of channels by which a star may evolve. Whilst the majority are likely the result of close binary evolution, evidence is growing for a significant population of short-period double-white dwarf binary stars (DWDs) that merge within a Hubble time. Determining the difference between the progenitor of an RCB star star and the progenitor of a Type Ia supernova (SN Ia) or between the progenitor of a single hot subdwarf and the progenitor of an AM CVn binary is crucial for understanding both the local stellar population and the cosmological distance ladder. We therefore propose a programme to discover the connection between evolved close binaries, double white dwarf mergers and the Galactic zoo of low-mass hydrogen-deficient stars. This programme focuses on the physics of quiet extreme phases of stellar evolution, and on transport processes within evolved low-mass stars. It has wider implications for the mostly unseen Galactic population of white dwarfs and the associated gravitational wave foreground, as well as for identifying the progenitors of SN Ia. In the following: the program of work for which the PDRA will be recruited is highlighted in blue. The PDRA will be encouraged to contribute to other strands of the programme. I. White Dwarf Mergers, Helium Stars, Hot Subdwarfs and SN Ia. Double white dwarf bina- ries (DWDs) have long been predicted to form as a consequence of close-binary star interaction (Webbink 1984, ApJ 277, 355), and short-period compact binaries have long been predicted to lose angular momentum by gravitational-wave radiation (Landau & Lifshitz 1958, The Classical Theory of Fields). However, only recently have observations unearthed increasing numbers of DWD binaries with significant numbers having periods short enough to merge within a Hubble time (eg SPY: Napiwotzki et al. 2007 ASPC 372, 387 et seq.; ELM: Brown et al. 2010, ApJ 723, 1072 et seq.); depending on mass ratio, progenitor candidates for AM CVn, RCB, hot subdwarf and Type .Ia supernova progenitors have been identified. To date, no DWDs having a combined mass clearly in excess of the Chandrasekhar mass are known, although this should represent a significant channel for SN Ia (eg Webbink 1984, Wang & Han 2012, NewAR 56, 122, Dan et al. 2014, MN 438, 14). Binary-star population-synthesis (BSPS) calculations give a range of estimates for the galactic DWD merger rate , including the superChandrasekhar fraction ; as one example typical of other recent work, Yu & Jeffery (2010, A&A 521, 85) estimate these to be = 0.0027/y and = 0.0013/y respectively, most coming from relatively recent star formation. The latter causes problems since RCBs, SN Ia and hot subwarfs are considered to belong to old populations, usually the bulge or thick disk. However, the bigger question for modern astronomy concerns the conditions a DWD must satisfy to explode as a SN Ia ­what are the lower mass limits for the components? Our approach involves identifying and studying products of DWD mergers which have clearly failed to explode and have thus left a longer-lived and observable remnant.
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Figure 1: Temperature-gravity diagram for part of the low-mass hydrogen-deficient star zoo, showing the hydrogen (H-MS) and helium main-sequences (He-MS), the horizontal-branch (HB), R CrB variables (RCB), extreme helium stars (EHe), hot subdwarfs of three types (He-sdB, He-sdOand He-sdO+), central stars of planetary nebulae ([WC-L] and [WC-E]) and the white dwarf sequence: PG11559 ­ DO ­ DB­DQ.

Covering a full range of possible donor and accretor masses, Dan et al. (2014) surveyed the structure of DWD mergers immediately after the dynamical phase, and identify possible outcomes with progenitor mass ranges, including hot subdwarfs, RCB stars and the boundaries above which explosions (eg SN Ia) should occur. Following Webbink (1984) and others, we had peviously identified specific CO+He WD mergers with RCB and EHe stars on the basis of 1d single-star evolution models (Saio & Jeffery 2002, MN 333, 121; Jeffery et al. 2011 MN 414, 3599). We also demonstrated that equal-mass He+He WD mergers will produce helium-rich hot subdwarfs, with the surface C/N ratio being an indicator of total mass (Zhang & Jeffery 2012a MN 419, 451, Fig. 5). Our models are 1d simulations of postmerger evolution, relying on ad hoc assumptions about immediate post-merger structure but providing a more detailed description of the resulting stars. Recent 3d hydro calculations including nucleosynthesis give new descriptions for the merger, which include a hot corona and Keplerian disk (Loren-Aguilar et al. 2009, A&A 500, 1193; Staff et al. 2012, ApJ 757, 76; Dan et al. 2014). These will be incorporated into our 1d simulations in order to: 1) make new calculations of CO+He DWD post-merger evolution including a hot coronal component, and allowing for improved treatments of convective mixing above the helium- burning shell, as this seriously affects the resultant chemical composition (cf. Lithium: Longland et al. 2012 ApJ 737, L34). These calculations will allow us to compare theoretical masses and lifetimes of EHe and RCB stars with observation. Determining the up- per mass boundary for merger survival em8


pirically provides an indirect test of the SN Ia predictions. Lifetimes tell long a post-merger star may be luminous but non-variable, i.e. before it the RCB phase. Current theory suggests there should be at least ten nonRCB hydrogen-deficient post-merger giants for every variable RCB star Jeffery 2002). Where and what are they?

us how reaches variable (Saio &


Figure 2: Evolution tracks for He+He DWD post-merger evolution with masses shown. The dotted track includes the formation of a hot corona on a timescale of hours and accretion from a Keplerian disk over 104 y. Post-merger evolution shows inward-burning helium shell flashes until the star becomes an He-sd where its life- time is 108 y after which it becomes a WD or an RCB star: Zhang & Jeffery 2012a.

2) make new He+He DWD calculations for unequal mass ratio cases, since it is the grav- itational potential depth and not the total mass which controls the temperature and hence nucleosythesis during the merger (Dan et al. 2014). Is it
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Figure 3: Elemental abundances for three intermediate He-sd's relative to solar values. Average abundances for He-sd's and normal sdBs are indicated by a mean (symbol) and range (line): Naslim et al. 2013, MN 434, 1920.

possible that a larger fraction of He+He DWD mergers could expand to become RCB stars than the 1% we found already? 3) test the physics assumed for the disk-accretion phase. An unresolved question addresses the post-merger lifetime of the Keplerian disk within the expanding envelope. If long, then our 1d calculations must account for injection of disk material into the appropriate part of the envelope in order to correctly model the surface chemistry. These simulations are not easy; we have some experience (Zhang & Jeffery 2012a,b MN 426, L81, Zhang et al. 2014, MN 445, 660) using MESA (Paxton et al. 2011, ApJS 192, 35). Using these or other appropriate techniques, the PDRA will attempt to follow post-merger evolution through to the WD phase for a number of cases, with particular reference to surface composition and pulsation stability. He may also attempt new BSPS calculations to better resolve the DWD progenitor populations, in particular by identifying which channels produce different classes of DWD binaries. Depending on the PDRA recruited, these tasks will take between 12 and 24 months. II. Heavy Metal and the Surface Composition of Hot Subdwarfs. Real stars can be connected with their previous history using a) surface composition ­ the archÔological record, b) duplicity ­ for mass, and c) pulsations ­ for mass and internal structure. As implied above, the products of DWD mergers are likely to be chemically peculiar. So, also, are many products of other binary-star interactions. Can we distinguish the product of a DWD merger from the product of other singleor binary-star channels? This question is important for both RCB/EHe stars (Jeffery et al. 2011) and for hot subdwarfs, which can be produced by several possible evolution channels. Using observed surface composition as a tracer of stellar
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evolution requires modelling nucleosynthesis and convective mixing. In high-gravity stars such as hot dwarfs, subdwarfs and white dwarfs, it also requires understanding radiative levitation and diffusion. He+He DWD mergers should produce an extremely helium-rich hot subdwarf (Hesd), essentially a low-mass helium star, also enriched in nitrogen and/or carbon. However the majority (90%) of hot subdwarfs are helium-deficient, poor in light elements and enriched in most elements heavier than calcium, although their underlying structure is also that of a low-mass helium star, but with a thin hydrogen envelope (Heber 2009, ARAA 47, 211). The fact they have some hydrogen can be understood in terms of a different binary-star history ­ mass-exchange or common-envelope ejection from a red-giant close to core-helium ignition. The fact that they are helium-deficient is explained by an equilibrium between gravitational and radiative forces, whereby ions diffuse to regions where the specific radiative support is a maximum. Thus helium and other light elements sink. So why do any He-sd's remain? Some 5% of sd's are extremely helium-rich (< 1% hydrogen by number) and 5% are intermediate (1-90% hydrogen). Do these stars have different histories, or are they transition objects? When did the majority become helium poor and how long did it take? Our first hypothesis is that extreme He-sd's are indeed the product of He+He WD mergers; there is simply no hydrogen left to float upwards to produce a genuine sdB star (Naslim et al. 2010 MN 409, 528, Zhang & Jeffery 2012a), and almost none are binaries1. Our second is that intermediate He-sd's are proto-sdB stars. Since helium ignition in a red-giant core is off-center, an sdB star approaches the zeroage horizontal branch via a series of shell-helium flashes (cf. Fig. 5). The proto-sdB star should therefore be more luminous than the average sdB star, and should have a surface in transition between that exposed by removal of the red- giant envelope and the helium-poor mixture described above. In fact, the intermediate He-sd's show a wide range of helium abundance, lower surface gravities, negligible rotation, some are in binaries and a few show an extraordinary (â104) excess of heavy metal (Fig.6). We argue that radiative levitation is rearranging the surface chemistry, helium is still sinking and other ions are being concentrated into equilibrium locations which coincide with the line-forming layer of the photosphere. We shall test these hypotheses by: 1) developing the treatment of diffusion in our stellar atmosphere models (Behara & Jeffery 2006) and in stellar interior models using the diffusion module in MESA (Paxton et al. 2011) or otherwise, 2) computing the diffusion timescales for various elements in realistic proto-sdB models to test for a dynamic surface composition in proto-sdB stars, 3) determining formation of In direct sup optical and
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how much hydrogen may be present before diffusion a helium-poor atmosphere ­ testing the first hypothesi port of these theoretical investigations, we will obtain ultraviolet spectra to identify and analyse other

triggers the s, high-quality heavy-metal

The exception is the He-sd+He-sd PG1544+488, possibly a product of double-core envelope ejection (Sener & Jeffery 2014, MN 440, 2676)

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intermediate He-sds, and search for systematic trends and evidence of photospheric stratification.

III. Hot-Star Pulsations under the Microscope. A consequence of reducing the hydrogen content or of increasing the metal content in a stellar envelope is that pulsations are more readily excited. EHes, "normal" sdBs and He-sds all include stars which pulsate, either radially or non-radially making it possible to measure their masses and explore their internal structure, history and physics. Some of these, like the intermediate He-sd LS IV-14.116 (Ahmad & Jeffery 2005, A&A 437, L51; Green et al. 2011, ApJ 734, 59; Jeffery 2011, IBVS 5964) pulsate in unexpected modes, whilst asteroseismic results for many sdB stars observed by Kepler are ambiguous. The PDRA will therefore: 1) examine the asteroseismic properties of post-merger and proto-sdB models computed above ­ and hence test whether their predicted internal properties match observations (up to 12 months). As well as the wider questions concerning the origins of SN Ia, the fate of DWD mergers, the origin and structure of hot subdwarfs and the physics of transport processes in extreme stars, the entire programme impacts on a parallel theoretical and observational study of pulsation in hydrogen-deficient remnants. The latter includes new non-linear pulsation models for EHes and He-sd's, coupled to a selfconsistent treatment of radiative transfer in the stellar atmosphere (cf. Jeffery et al. 2014, APCS 479, 369) being developed to interpret several years of photometry and high-resolution spectroscopy of the long-period pulsations in EHe stars and also Subaru, VLT and SWIFT datasets for the pulsating EHe V652 Her and He-sd LS IV -14.116. The resulting mass and internal structure measurements will, inter alia, test the post-CO+He DWD merger models and set an upper mass for failed DWD SN Ia progenitors, and will address the question of the driving mechanism in LS IV -14.116.

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THE GOVERNORS OF ARMAGH OBSERVATORY AND PLANETARIUM Job Advertisement Armagh Observatory Postdoctoral Research Assistant - Stellar Evolution Modelling The Armagh Observatory invites applications for a Postdoctoral Research Assistant in Astrophysics. The successful applicant will work with Prof Simon Jeffery on developing models for the evolution, structure and fate of merged double white dwarf stars. Objectives include a study of the conditions necessary to avoid a supernova explosion, and a comparison with observed properties of evolved stars including hot subdwarfs.
Situated near the centre of Armagh, Northern Ireland, the historic Armagh Observatory has active research groups working in stellar, galactic, solar and solarsystem astronomy. This is a fixed-term position funded by the UK Science and Technology Facilities Council. Applicants should have a recent PhD in astrophysics with a strong research record in relevant areas, including substantial experience in the use of major stellar evolution software. The position is available for three years, and can commence on 1 April 2015 or as soon as possible thereafter.
Starting salary will be in the range ¸27,274 - ¸29,784 depending on experience.
Full details of the post are available at: star.arm.ac.uk/jobs. Applicants should submit a CV, statement of research interests, and the names of three referees, who may be approached, to: Human Resources, Armagh Observatory, College Hill, Armagh BT61 9DG, UK (hr@arm.ac.uk), to arrive no later than 3pm on 13 March 2015*. Enquires about the position can be directed to Prof Simon Jeffery: csj@arm.ac.uk .


*Deadline extended from 2 March 2015

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