Spiralna struktura galaksij

Maruša Žerjal

Fakulteta za matematiko in fiziko

torek, 25. 5. 2010, ob 15h v F7

Spiralne galaksije spadajo v skupino največjih in najštevilčnejših svetlih galaksij. S svojimi značilnimi spiralnimi rokavi, ki so področja rojevanja novih zvezd, predstavljajo enega izmed najlepših pojavov v naravi. Preko osnov galaktične dinamike in orbit zvezd ter poznavanja pogojev za nastanek novih zvezd se bomo lotili razlage pojava spiralnih rokavov. Ovrgli bomo hipotezo o snovnih rokavih, ki predpostavljajo fiksno sestavo posameznega rokava in predstavili Lin-Shu-jevo teorijo gostotnih valov. Slednja pojav opisuje preko perturbacijske teorije orbit, ki je povezana z nastankom področij, kjer je gostota plinov in prahu večja od okoliške za 10-20%. Znotraj spiralnih rokavov je torej velika verjetnost, da bo izpolnjen Jeansov kriterij in da bo prišlo do rojevanja novih vročih in svetlih zvezd, ki osvetljujejo rokave. Ker je življenjska doba teh zvezd veliko manjša v primerjavi z njihovim obhodnim časom okoli jedra galaksije, v prostoru med rokavi najdemo le manj svetle zvezde. Na koncu si bomo pogledali še rezultate nekaterih meritev, ki razkrivajo, da ima naša Mlečna cesta obliko spiralne galaksije s prečko.

Lyman-alfa prehod v spektrih oddaljenih kvazarjev

Vid Iršič

Fakulteta za matematiko in fiziko

torek, 11. 5. 2010, ob 15h v F7

Zaradi širjenja vesolja je svetloba iz oddaljenih kvazarjev Dopplersko premaknjena, kar opazimo v opazovanih spektrih. Oblaki vodikovih atomov v osnovnem stanju med nami in kvazarjem bodo absorbirali le svetlobo določene valovne dolžine in v spektru pustili odtis v obliki absorbcijske črte. Ker so ti oblaki na manjši razdalji od nas kot kvazar, se bodo njihove absorbcijske črte premaknile za manjši Dopplerjev faktor. V opazovanih spektrih kvazarjev opazimo črte prehodov iz osnovnega stanja vodika (Lymanove serije), prevladuje pa prehod iz n = 1 v n = 2 stanje, ki mu pravimo Lyman-alfa prehod. Z merjenjem takšnih absorbcijskih črt lahko raziskujemo naravo medgalaktične snovi, zaradi velikh razdalj pa nam omogočajo tudi določanje kozmoloških parametrov: evolucijo temne snovi H(z), amplitudo in obliko spektra moči fluktuacij gostote P(k) in gaussove začetne pogoje. Ker so velikosti absorberjev tipično manjše od galaksij, lahko opazujemo fiziko na nizkih velikostnih skalah v zgodnjem vesolju. To nam med drugim omogoča določanje nevtrinskih mas \Omega_\nu, vlogo sterilnih nevtrinov, vroče temne snovi in inflacijskih modelov (n_s, \alpha) preko spektra moči P(k). Vedno pomembnejšo vlogo pa igra tudi v določanju skale barionskih akustičnih oscilacij.

Observations of particle acceleration in gamma-ray bursts

Peter Curran

Mullard Space Science Laboratory, University College London

torek, 4. 5. 2010, ob 15h v F7

I will introduce Gamma-ray bursts (GRBs), which are the most luminous and distant observable explosions in the Universe. I will explain how they are produced by a collimated ultra-relativistic outflow (a jet) emerging from the deep potential well of a massive star that has collapsed into a black hole. Within these jets, shock fronts accelerate charged particles to the relativistic speeds required so that they may radiate electromagnetically via synchrotron emission. Fundamental to the acceleration process is the electron energy distribution index and here I will examine the constraints placed on the distribution index by the observed properties of gamma-ray bursts. Given that the acceleration process applies to many astronomical jet sources, such as those from X-ray binaries and AGN (as well as particle acceleration in the solar wind and supernovae, and the acceleration of cosmic rays), the accurate measurement of the distribution of p is of fundamental importance to differentiate between the possible theories of electron acceleration at any relativistic shock front.

Most probable distances to RAVE stars assuming a standard stellar evolution scenario

Tomaž Zwitter

Fakulteta za matematiko in fiziko

torek, 23. 3. 2010, ob 15h v F7

The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way which already collected over 400.000 spectra of ∼330.000 different stars. We use the subsample of spectra with spectroscopically determined values of stellar parameters to determine the distances to these stars. The list currently contains 235.064 high quality spectra which show no peculiarities and belong to 210.872 different stars. The numbers will grow as the RAVE survey progresses. The public version of the catalog will be made available through the CDS services along with the ongoing RAVE public data releases. The distances are determined with a method based on the work by Breddels et al. (2010). Here we assume that the star undergoes a standard stellar evolution and that its spectrum shows no peculiarities. The refinements include: the use of either of the three isochrone sets, a better account of the stellar ages and masses, use of more realistic errors of stellar parameter values, and application to a much larger dataset. The derived distances of both dwarfs and giants match within ∼20 % to the astrometric distances of Hipparcos stars and to the distances of serendipitously observed members of open and globular clusters. Multiple observations of a fraction of RAVE stars show that repeatability of the derived distances is even better, with a dispersion of <12 %. RAVE dwarfs are ∼300 pc from the Sun, and giants are at distances of 1 to 2 kpc, and up to 10 kpc. This places the RAVE dataset between the more local Geneva-Copenhagen survey and the more distant and fainter SDSS sample. As such it is ideal to address some of the fundamental questions of Galactic structure and evolution in the pre-Gaia era. Individual applications are left to separate papers, here we show that the full 6-dimensional information on position and velocity is accurate enough to discuss the vertical structure and kinematic properties of the thin and thick disks.

The Mid-Infrared Luminosities of Normal Galaxies over Cosmic Time

Urtzi Jauregi

Fakulteta za matematiko in fiziko

torek, 16. 3. 2010, ob 15h v F7

Daniel D. Kelson (Carnegie Observatories), Bradford P. Holden (UCO/Lick Observatories):

Modern population synthesis models estimate that 50% of the restframe K-band light is produced by TP-AGB stars during the first Gyr of a stellar population, with a substantial fraction continuing to be produced by the TP-AGB over a Hubble time. Between 0.2 and 1.5 Gyr, intermediate mass stars evolve into TP-AGB C stars which, due to significant amounts of circumstellar dust, emit half their energy in the mid-IR. We combine these results using published mid-IR colors of Galactic TP-AGB M and C stars to construct simple models for exploring the contribution of the TP-AGB to 24 micron data as a function of stellar population age. We compare these empirical models with an ensemble of galaxies in the CDFS from z=0 to z=2, and with high quality imaging in M81. Within the uncertainties, the TP-AGB appears responsible for a substantial fraction of the mid-IR luminosities of galaxies from z=0 to z=2, the maximum redshift to which we can test our hypothesis, while, at the same time, our models reproduce much of the detailed structure observed in mid-IR imaging of M81. The mid-IR is a good diagnostic of star formation over timescales of ~1.5 Gyr, but this implies that on-going star formation rates at z=1 may be overestimated by factors of ~1.5-6, depending on the nature of star formation events. Our results, if confirmed through subsequent work, have strong implications for the star formation rate density of the universe and the growth of stellar mass over time.

Članek: http://arxiv.org/abs/1003.1420

Nekozmološke hitrosti in supernove tipa Ia

Vid Iršič

Fakulteta za matematiko in fiziko

torek, 2. 3. 2010, ob 15h v F7

Že od prvih opazovanj supernov so bili ti svetli dogodki predlagani kot standardne svetilke za kozmološke meritve. Mnogo raziskav supernov tipa Ia (SNe Ia) je v nadaljnih letih razkrilo, da se vesolje širi pospešeno zaradi novega in nepričakovanega gravitacijskega pojava, kar dandanes imenujemo temna energija. Iz opazovalnih podatkov SNe Ia lahko izmerimo željene kozmološke parametre s pomočjo Hubblovega zakona (H_0, \Omega_m, \Omega_\Lambda, w). V zadnjih letih je postalo očitno, da korelacije med nekozmološkimi hitrostmi (’peculiar velocities’) supernov nezanemarljivo prispevajo k rezultatom in tako predstavljajo možnost določanja spektra moči snovi (P_m(k)) ter preko njega kozmoloških parametrov, ki niso povezani s širjenjem vesolja (\Omega_b, n_s, \sigma_8).

Relativity and positioning systems

Pacome Delva

European Space Agency

torek, 9. 2. 2010, ob 14h v F6

The classical concept of positioning system for a Global navigation satellite system (GNSS) would work ideally if all satellites and the receiver were at rest in an inertial reference frame. But at the level of precision needed by a GNSS, one has to consider curvature and relativistic inertial effects of spacetime, which are far from being negligible.

In this talk I will present two very different ways of including relativity in positioning systems: one way is to keep the newtonian conception of absolute time and space, and add a number of corrections depending on the desired accuracy; another way is to use a relativistic positioning system. This is a complete change of paradigm, as the constellation of satellites is described in a general relativistic framework.

Model nevtronske zvezde

Maruša Žerjal

Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 22. 12. 2009, ob 14h v F6

Odkritje nevtronskih zvezd oz. pulzarjev leta 1967 predstavlja eno največjih astronomskih odkritij 20. stoletja, saj so ti kompaktni objekti primer ekstremnih razmer, kakršnih v laboratorijih ne dosežemo. Še posebej so zanimivi zato, ker gostota snovi v središču zvezde s tipičnim radijem 10\;\mathrm{km} za \sim 10 \times preseže gostoto atomskega jedra, kritična temperatura za superprevodnost oz. superfluidnost pa je 10^{10}\;\mathrm{K}. Gostota magnetnega polja zvezde doseže vrednosti tudi do 10^{11}\;\mathrm{T}. Ker laboratorijskih eksperimentov ni, zanesljivost enačbe stanja z gostoto pada, na srečo pa so nam na voljo opazovanja pulzarjev.

Konstruirali bomo preprost model nevtronske zvezde, sestavljene iz degeneriranega plina elektronov in nevtronov. Opisali bomo plastovitost modela in se dotaknili procesa nevtronizacije, ki snov obogati z nevtroni. Na koncu bomo opažene pojave na pulzarjih povezali s teoretičnimi napovedmi modela.

Zvezdna kopica okoli črne luknje v centru Galaksije

Janez Kos

Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 15. 12. 2009, ob 14h v F6

V začetku prejšnjega desetletja so odkrili nekaj nenavadnih objektov na območju, kjer naj bi bil center naše Galaksije. S prihdom aktivne optike se je opazovalcem razkrila gosta kopica zvezd, katerih članice krožijo okoli masivne črne luknje. Ekstremni pogoji, ki vladajo v centru, so vzrok nekaterim dogodkom, ki jim nismo priča nikjer drugje v Galaksiji (na primer pogosti trki zvezd). Opazovanje dinamike nam da natančno vrednost mase črne luknje in porazdelitev mase v okolici. Vsak odgovor pa poraja mnogo vprašanj. Tako še vedno ni jasno, kako je kopica nastala in zakaj jo sestavlja nenavadno število vročih orjakinj. Na astrodebati bom povedal nekaj o opazovanju dinamike v Galaktičnem centru, o parametrih, ki jih bomo lahko izmerili z dolgotrajnejšimi opazovanji, opisal bom pogoje, ki vladajo v centru, ter razložil nekaj modelov, ki poskušajo opisati nastanek kopice.

Spektroskopske dvojne zvezde v pregledu neba RAVE

Gal Matijevič

Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 8. 12. 2009, ob 14h v F6

Od začetka delovanja leta 2003 je bilo v okviru projekta RAVE posnetkih že okoli 400.000 zvezdnih spektrov. Poleg glavnega cilja meritve radialnih hitrosti prek premika spektralnih črt je mogoče iz zajetih podatkov izmeriti še marsikaj zanimivega. Težave nastopijo, če slepo predpostavimo, da so vsi spektri brez napak in pripadajo običajnim zvezdam. Izkaže se, da se znaten delež spektrov slabo ujema z modelom, kar pomeni, da je potrebno takšne spektre obravnavati posebej. Ogledali si bomo, kako v velikem vzorcu relativno zanesljivo odkrijemo vse “nepravilne” spektre in se nato osredotočili na dvojne zvezde. S pomočjo simulacije bomo preverili zanesljivost klasifikacijske metode in hkrati ugotovili, kakšna dvozvezdja je sploh mogoče odkriti. Ob koncu bomo naredili še pregled novo odkritih dvojnic v zajetih podatkih in si še ogledali, kako iz opazovanih spektrov določimo nekatere zvezdne parametre.

Aktualna področja astronomije

Gal Matijevič

Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 24. 11. 2009, ob 14h v F6

Ob pregledovanju prispevkov na spletni strani astro-ph (arxiv.org/archive/astro-ph/) je mogoče razbrati, da se članki z nekaterih področji astronomije pojavljajo pogosteje kot z drugih. Bralcu se hitro zastavi vprašanje, ali je morda za uspešno delo potrebno slediti trendu? Na debati bomo skupaj poskusili najti odgovor, poleg tega pa še skušali ugotoviti, kako pomembna je izbira področja na začetku astronomske kariere in kako izbrati primeren podiplomski študij ali temo za seminarsko nalogo.

Vabljeni seveda vsi, še posebej pa izkušenejši kolegi in študenti, ki jih morda zanima nadaljevanje študija astronomije.

Mapping the Spacetime Metric with a Global Navigation  Satellite System

Uroš Kostić

Univerza v Ljubljani, Fakulteta za matematiko in fiziko

torek, 17. 11. 2009, ob 14h v F6

1. We propose to give a complete set of exact analytic solutions of geodesics, both time-like and light-like, in the Schwarzschild space time.

2. The space-time metric in the vicinity of the Earth is a solution of Einstein’s equations, which can be described as a perturbed Schwarzschild metric. Using the formalism similar to that of Regge-Wheeler, Zerilli, Chandrasekhar and others we propose to search for the canonical perturbations best describing the gravitational field in the vicinity of the Earth, in the coordinate frame centered at the center of mass of the Earth, taking into account the gravitational field of the Moon, Sun, Jupiter and rotation of the Earth.

3. As the space-time metric about the Earth can be described as a perturbed Schwarzschild metric, we propose to develop first order Hamiltonian perturbation theory to describe geodesics in the perturbed Schwarzschild metric.

Circumstellar Environments of Recurrent Novae

Valerio A.R.M. Ribeiro

Astrophysics Research Institute, Liverpool John Moores University

petek, 2. 10. 2009, ob 13h v F3

I report recent studies of two novae, RS Ophiuchi (a well known Recurrent Nova) and V2491 Cygni (which is suspected to be Recurrent). RS Oph has been widely studied, and suggested by some astronomers as a SN Type Ia progenitor candidate. I will discuss in particular our HST imaging and optical spectroscopy, combined with modelling, of the expanding nebular remnant of its latest outburst in 2006. Nova V2491 Cyg is a strong RN candidate and I will discuss our initial work on optical spectra taken during its 2008 outburst with the Liverpool Telescope which may provide us with clues to its true nature. Overall, these results emphasise the need for simultaneous ground- and space-based multi-frequency observations if we are to truly understand these systems and their wider implications to other astronomical objects.