The nature of the First Stars: the point of view of the neutron capture elements

dr. Gabriele Cescutti

National Institute for Astrophysics

Astronomical Observatory of Trieste

v sredo, 31. maja 2017, ob 12. uri v predavalnici F4 (Jadranska 19, Ljubljana)

In the last years our group has found that many chemical anomalies observed in very metal-poor halo stars in the light elements suggest the first stellar generations to have been fast rotators (spinstars). Recently, theoretical computations have found that spinstars can also play a role in the chemical enrichment of neutron capture elements providing a early contribution of s-process. By means of a stochastic chemical evolution model, it is possible to identify the spinstars s-process contribution as the missing component responsible for the spread in the ratio between light (Sr) to heavy (Ba) neutron capture elements. A specific distribution is predicted for the isotopic ratio of Ba in halo stars and this imprint could be the smoking gun of the role played by spinstars in the spread of [Sr/Ba] ratio. In this context, regarding the still unknown origin of the complementary r-process component, I present new constraints on the rate and time scales of r-process events, based on the recent discovery of the r-process rich stars in the ultra faint galaxy Reticulum 2.

Spektroskopija obrnjena na glavo

dr. Janez Kos

Sydney Institute for Astronomy, Univerza v Sydneyu, Avstralija

v ponedeljek, 29. maja 2017, ob 12. uri v predavalnici F6 (Jadranska 19, Ljubljana)

Moderni spektroskopi lahko hkrati opazujejo stotine ali tisoče zvezdnih spektrov. S pomočjo optičnih vlaken pripeljejo svetlobo zvezd v spektroskop in tako izkoristijo celoten razpon optike in ccd detektorja. Ker navadno potreba po kvantiteti prevlada nad kvaliteto, je take spektre težko obdelati, ne da bi zavrgli kar nekaj informacije.

Če zelo dobro poznamo optiko spektroskopa in optičnih vlaken, lahko obdelavo v celoti izpustimo. Namesto da bi spektre obdelali in jih nato analizirali, lahko proces obrnemo: izdelamo sintetične spektre, modele, ki predstavljajo spektre v rangu nekih parametrov (temperatura, kovinskost zvezde, itd), ki jih iščemo, ter jih konvuliramo z dobro znano prenosno funkcijo optike spektroskopa in vlaken. Tako lahko iz modela spektra zgradimo ‘’surov” neobdelan spekter in ga primerjamo z izmerjenim, prav tako neobdelanim spektrom. Prednost te metode je, da izboljša resolucijo nad nominalno vrednost spektroskopa, pravilno uteži spektre, ki se na surovi sliki prekrivajo in omogoča manj konzervativen dizajn bodočih spektroskopov. Slabost je seveda ta, da je metoda računsko zelo zahtevna in terja dobro poznavanje optike inštrumenta.

Pokazal bom kako smo natančno parametrizirali optiko 4 meterskega teleskopa AAT in spektroskopa Hermes ter predstavil prve rezultate nove metode, ki sem jo razvil.

Hydrodynamical simulations: AGN feedback and its influence on the ICM of Galaxy Clusters

dr. Elena Rasia

National Institute for Astrophysics

Astronomical Observatory of Trieste

v sredo, 24. maja 2017, ob 12. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Powerful phenomena determine the energy of Galaxy Clusters from sub-pc scales to Mpc distances. Via high-resolution, cosmological and hydrodynamical simulations we study how the main properties of the ICM (entropy, temperature, pressure, and metallicity) are influenced by the presence of the central AGN. We compare the results from runs with AGN feedback with those with “exclusively” stellar feedback to enhance the importance of the powerful AGN source. We found that the AGN are key to create the observed metal profile and to produce clusters which are cool cores. Observationally, cool-core clusters have precise physical characteristics: they are found in regular clusters with a symmetric and peaked surface brightness distribution and have a peaked metal profile. Our numerical models are able to reproduce most of the observed properties, thus can be used to derive some predictions on the evolution of these systems.

Supernovae seen through gravitational telescopes

Tanja Petrushevska

Department of Physics, Faculty of Science, Stockholm University

The Oskar Klein Centre for Cosmo Particle Physics, Sweden

v sredo, 26. aprila 2017, ob 12. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Galaxies, and clusters of galaxies, can act as gravitational lenses and magnify the light of objects behind them. The effect enables observations of very distant supernovae, that otherwise would be too faint to be detected by existing telescopes, and allows studies of the frequency and properties of these rare phenomena when the universe was young. Under the right circumstances, multiple images of the lensed supernovae can be observed, and due to the variable nature of the objects, the difference between the arrival times of the images can be measured. Since the images have taken different paths through space before reaching us, the time-differences are sensitive to the expansion rate of the universe. One class of supernovae, Type Ia, are of particular interest to detect. Their well known brightness can be used to determine the magnification, which can be used to understand the lensing systems. I will also report our discovery of the first resolved multiply-imaged gravitationally lensed supernova Type Ia.

Eksoplanetna odkritja pokvarjenega Keplerja, misija K2

Teo Močnik

Astrophysics Group, Keele University, Staffordshire, UK

v sredo, 19. aprila 2017, ob 12. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Vesoljski teleskop Kepler je zaslužen za največ potrjenih odkritij eksoplanetov doslej. To je bilo možno zaradi večletnega praktično neprekinjenega in zelo natančnega merjenja svetlosti več kot 100.000 zvezd v ozvezdju Laboda. Odkar se je maja 2013 pokvarilo še drugo od skupno štirih Keplerjevih reakcijskih koles, stabilna usmeritev v ozvezdje Laboda več ni bila mogoča, kar je pomenilo konec opazovalne misije Kepler. Po enem letu iskanja rešitev in testiranj je NASA leta 2014 potrdila načrt nadaljne uporabe teleskopa Kepler, poimenovan misija K2. Od takrat teleskop opazuje polja vzdolž ekliptike, kar omogoča zadostno natančnost usmeritve kjub zgolj dvema delujočima reakcijskima kolesoma.

Najprej bom predstavil postopek obdelave K2 opazovanj s katerim odpravimo napake v svetlobnih krivuljah kot posledice nepopolne stabilnosti usmeritve teleskopa. Nato se bomo sprehodili skozi galerijo zanimivih planetnih sistemov, ki so bili opazovani v okviru misije K2 v 1-min kadenčnem načinu. V sistemih WASP-85, Qatar-2 in WASP-107 smo zaznali okultacije zvezdnih peg, ki razkrijejo natančno rotacijsko periodo matične zvezde in dragocen podatek (ne)poravnanosti osi vrtenja zvezde z orbitalno osjo planeta. Vročemu Jupitru Qatar-2b je uspelo popačiti obliko zvezde gostiteljice v elipsoid, kar je edinstvena meritev v okviru misije K2. WASP-118b pa je prvi odkriti eksoplanet, ki kroži okoli pulzirajoče zvezde tipe Gamma Doradus. Omenil bom še WASP-75 z navideznim mimoletom svetlega asteroida in WASP-55.

Ali je temna snov zamegljena?

dr. Vid Iršič

Institute for Advanced Studies, Princeton, University of Washington, Department of Astronomy

v četrtek, 22. decembra 2016, ob 15. uri v predavalnici F2 (Jadranska 19, Ljubljana)

Standardni kozmološki model, ki vsebuje hladno temno snov (CDM), neverjetno uspešno opiše razvoj vesolja na velikih razdaljah. Na razdaljih, ki so tipične za galaksije in jate galaksij (< 10 Mpc) pa model nima močne podpore meritev. Eden od načinov, kako razložiti razlike med napovedmi kozmološkega modela in meritvami, je v tem kaj temna snov pravzaprav je. Meritve porazdelitve gostote v Lyman-alpha gozdu najbolj omejujejo raznovrstne modele temne snovi.

S pomočjo podatkov ESO raziskave X-Shooter XQ-100, lahko izmerimo Lyman-alfa gozd v spektrih 100 kvazarjev (QSO) v podobnem obsegu rdečih zamikov kot pregled neba SDSS (z=3 - 4.2), vendar z veliko večjo natančnostjo na majhnih razdaljah (k_max ~ 6 h/Mpc). V tej predstavitvi bom povzel najnovejše rezultate in komentiral analizo podatkov. Skupaj z meritvami posameznih kvazarjev pri višjih rdečih zamikih, lahko podamo nove rezultate o modelih tople temne snovi (WDM). V glavnem delu predstavitve bom komentiral kako ti rezultati vplivajo na rešitev nerešenih vprašanj standardnega kozmološkega modela.

Kako zvezde dobijo svoj EMŠO

dr. Janez Kos

Sydney Institute for Astronomy, Univerza v Sydneyu, Avstralija

v sredo, 21. decembra 2016, ob 11. uri v predavalnici F6 (Jadranska 19, Ljubljana)

Zastopanost elementov v atmosferah zvezd je lastnost, ki jo vsaka zvezda dobi ob svojem rojstvu. Zvezde se rodijo iz oblaka plina in prahu, ki je kemično homogen. To drži dokler prva zvezda v tako nastali kopici ne eksplodira kot supernova in poruši kemično homogenost. To se navadno zgodi po tem, ko se je porabil že ves material, ki je na voljo za tvorjenje zvezd, tako da vse zvezde v kopici nastanejo iz enakega materiala. Ko se zvezde kopice po nekaj 100 milijonih let porazgubijo po Galaksiji, je njihov kemični podpis edino kar jih še povezuje med sabo. Če znamo dovolj natančno izmeriti zastopanost večjega (~30) števila elementov v zvezdah, lahko najdemo zvezde, ki so bile nekoč članice iste kopice.

GALAH je spektroskopski pregled zvezd, ki namerava narediti prav to. Predstavil bom metodo in prve rezultate iskanja kemičnih podpisov zvezd. S 13 do sedaj opazovanimi elementi in ~150.000 zvezdami smo omejeni na znane kopice. Demonstriral bom kako s kemičnimi podpisi najdemo zvezde kopice in kako smo na ta način našli dve novi zvezdi, ki sta pred kratkim pobegnili iz Plejad.

KiDS pregled neba in šibko gravitacijsko lečenje

Andrej Dvornik

Observatorij v Leidnu, Univerza v Leidnu, Nizozemska

v torek, 20. decembra 2016, ob 11. uri v predavalnici F5 (Jadranska 19, Ljubljana)

KiDS pregled neba, ali Kilo Degree Survey, je velik pregled južnega neba v optičnih valovnih dolžinah (baziran na VST teleskopu v Čilu), namenjen preučevanju temeljnih kozmoloških vprašanj in formacije galaksij s pomočjo šibkega gravitacijskega lečenja.

V tem predavanju bom prestavil tehnične aspekte pregleda neba, teleskopa na katerem se zajemajo originalne fotografije in obdelavo podatkov, ki temelji na natančnih meritvah oblik galaksij. Bolj podrobno se bom dotaknil novejših rezultatov – meritve kozmičnega lečenja in kozmoloških parametrov dobljenih iz te analize, meritev lastnosti med temno snovjo in galaksijami ter t.i. assembly bias-a. Poudarek bo tudi na sami teoriji šibkega gravitacijskega lečenja in uporabi le tega.

Galaxy clusters from cosmological hydrodynamical simulations: the intracluster medium

dr. Veronica Biffi

University of Trieste and INAF – OATs

v sredo, 7. decembra 2016, ob 11. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Investigating cluster properties is crucial to validate our understanding of the main physical processes driving the formation and evolution of cosmic structures.

Cosmological hydrodynamical simulations, able to successfully reproduce many of the observed properties of galaxy clusters, allow us to study their intrinsic structure, as well as their formation and evolution. In fact, the highly non-linear processes involved in the evolution of baryons within clusters require complex calculations that can only be undertaken via numerical hydrodynamical simulations. Numerical results ultimately allow us to interpret observational findings from current telescopes as well as to make predictions that we will be able to test with up-coming observational missions (e.g. Athena, eRosita, etc.).

Here, I will present results on the thermo-dynamical and chemical properties of a sample of galaxy clusters from cosmological simulations that treat a large variety of physical processes, including AGN feedback.

Galaksije kot vir kozmične reionizacije

dr. Jure Japelj

INAF – OATs (Tržaški astronomski observatorij), Italija

v sredo, 9. novembra 2016, ob 12. uri v predavalnici F3 (Jadranska 19, Ljubljana)

Oblaki medgalaktičnega plina v nam bližnjem Vesolju so bolj ali manj ionizirani, na kar opozarjajo številni eksperimenti. Začetek kozmične reionizacije predstavlja izredno pomembno stopnjo v razvoju Vesolja, saj sovpada z nastankom prvih zvezd in galaksij. Medtem ko lahko precej natančno določimo čas, ko je stopnja ionizacije vodika dosegla današnjo raven (rdeči premik ~6), pa imamo veliko več težav pri identifikaciji in razumevanju virov ionizirajočega sevanja. To predstavlja precejšen problem, saj je naše razumevanje fizike medgalaktične snovi in nastanka galaksij tesno povezano z natančnim poznavanjem lastnosti ionizirajočega sevanja. V predavanju bom predstavil trenutno stanje na področju raziskovanja virov sevanja, pri čemer se bom osredotočil na prispevek galaksij z visoko stopnjo nastajanja zvezd. Kolikšen je prispevek takšnih galaksij k sevanju v primerjavi z galaksijami z aktivnimi galaktičnimi jedri? Kako pomembne so pri tem (zelo) temne galaksije? Zakaj je število galaksij, pri katerih neposredno opazimo ionizirajoče sevanje, tako majhno? Pokazal bom, kako pomembno vlogo pri odgovorih na zgornja vprašanja igrajo nedavni globoki pregledi neba, opravljeni z različnimi instrumenti (HST, VLT/MUSE).

On the Origin and Growth of Cosmic Magnetic Fields

dr. Alexander M. Beck

University Observatory Munich, Germany

v sredo, 19. oktobra, ob 12. uri v predavalnici F3 (Jadranska 19, Ljubljana)

The origin of magnetic fields in high-redshift and present-day galaxies is a long-standing problem. In this talk, we present a model for the seeding and evolution of magnetic fields in protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy self-consistently provide magnetic seed fields, which are subsequently amplified by compression, shear flows and random motions.

We present cosmological simulations with the GADGET code of Milky Way-like galactic halo formation using a standard LCDM cosmology and analyse the strength and distribution of the evolving magnetic field. We analyse the intrinsic rotation measure (RM) of the forming galactic halo over redshift. The mean halo intrinsic RM peaks between redshifts z=4 and z=2 and reaches absolute values around 1000 rad/m$^2$. Towards redshift z=0, the intrinsic RM values decline to a mean value below 10 rad/m$^2$. At high redshifts, the distribution of individual starforming and thus magnetized regions is widespread leading to a widespread distribution of large intrinsic Rms.

Our model for the evolution of galactic magnetic fields solves the joint problem of magnetic field seeding and subsequent amplification and distribution. The magnetic fields in galaxies are a direct consequence of the very basic processes of star and galaxy formation.

Cosmic-Ray Acceleration in galactic Interactions and its implications

prof. Tijana Prodanović

University of Novi Sad, Serbia

v sredo, 15. junija, ob 14. uri v predavalnici F5 (Jadranska 19, Ljubljana)

It has been shown that galactic interaction and mergers can give rise to large-scale tidal shocks that propagate through their gas. As a result, this can give rise to a new population of cosmic rays, additional to standard galactic cosmic rays present in star-forming galaxies. We investigate the impact of this tidal cosmic-ray population on the nucleosynthesis of lithium in interacting systems in the context of the cosmological lithium problem. Moreover we also demonstrate that the development of these interaction shock-waves and may also have far reaching consequences on our understanding of galactic evolution and star formation history, by affecting the far-infrared radio correlation observed in star-forming galaxies.

Relativistične N-body simulacije in njihova uporaba v kozmologiji

Mateja Gosenca

Univerza v Sussexu (UK)

v ponedeljek, 6. junija, ob 14. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Numerične simulacije delcev temne snovi se uporabljajo za modeliranje nastanka struktur v vesolju. Večina teh simulacij temelji na enačbah Newtonove mehanike. Za veliko problemov, ki jih takšne simulacije naslavljajo, je to odlična poenostavitev. Toda za modeliranje struktur, ki nastanejo zelo zgodaj, velikostnih skal blizu kozmološkega horizonta in relativističnih virov perturbacij, takšna aproksimacija ne zadostuje. Predstavila bom razvoj simulacij, ki namesto Newtonove gravitacije uporabljajo enačbe splošne relativnosti in hkrati delcem dovolijo relativistične hitrosti. Predvsem slednje je idealno za modeliranje tako imenovanih ultrakompaktnih minihalojev, ki predstavljajo edinstveno možnost detekcije prvobitnega spektra moči na majhnih velikostnih skalah. Poleg tega se s tem odpirajo vrata simulacijam modelov modificirane gravitacije, ki ponujajo eno od teoretsko najbolj zanimivih razlag pojava temne energije.

GALAH in difuzne medzvezdne črte

dr. Janez Kos

The University of Sydney, Sydney Institute for Astronomy

v sredo, 16. decembra 2015, ob 12. uri v predavalnici F3 (Jadranska 19, Ljubljana)

Na področju difuznih medzvezdnih črt smo dosegli velik preskok z opazovanji spektroskopskega pregleda neba RAVE; prvič je postal mogoč tri dimenzionalni vpogled v strukturo medzvezdne snovi tudi izven galaktične ravnine. RAVE je pred kratkim končal z opazovanji, začel pa se je nov pregled neba imenovan GALAH (GALactic Archaeology with Hermes), ki se lahko smatra za nekakšnega nadgrajenega naslednika RAVE-a.
GALAH ponuja spektre v štirih pasovih vidnih in bližnje IR svetlobe z resolucijo R=28 000 in razmerjem signala proti šumu nad 100. V astrodebati bom predstavil napredek in nove izzive pri opazovanju difuznih medzvezdnih črt v GALAH-ju. Govoril bom tudi o ostalih raziskovalnih projektih, ki potekajo v okviru projekta GALAH in o raziskovalnih priložnostih, ki so še vedno na voljo vsem, ki imajo/imamo dostop do GALAH-jevih spektrov.

Relativistic effects in Lyman-alpha forest

dr. Vid Iršič

ICTP Trieste, Italy

v torek, 6. oktobra 2015, ob 13. uri v predavalnici F4 (Jadranska 19, Ljubljana)

As the observations move out to larger scales the validity of a simple Newtonian description starts to come into question. Recent years have seen a lot of effort put into understanding and possibly measuring the signal of relativistic effects and/or weak gravitational lensing in the 2-point statistics of the galaxy clustering.
I will present the first study to extend the formalism to a different tracer of underlying matter at unique redshift range (z = 2 − 5). Furthermore, I will show a comprehensive application of the calculations to the Quasar-Lyman-alpha cross-correlation function. The results indicate that the signal of relativistic effects can be as large as 30% at Baryonic Acoustic Oscillation (BAO) scale, which is much larger than anticipated and mainly due to the large differences in density bias factors of our tracers.

A search for Galactic transients disguised as gamma-ray bursts

Aurora Clerici

University of Ferrara

v torek, 8. septembra 2015, ob 14. uri v predavalnici F6 (Jadranska 19, Ljubljana)

A significant fraction of cosmological gamma-ray bursts (GRBs) are characterised by a fast rise and exponential decay (FRED) temporal structure. This is not a distinctive feature of this class, since it is observed in many Galactic transients and is likely descriptive of a sudden release of energy followed by a diffusion process. We searched for possible Galactic intruders disguised as FRED GRBs in the Swift catalogue. We tested the degree of isotropy through the dipole and the quadrupole moment distributions. In addition, we searched for possible indicators of a Galactic origin among the spectral and temporal properties of individual GRBs. In this talk, we will present the results of our analysis.

For more information see http://arxiv.org/abs/1508.04898

Space coronagraphy with ESA

Prof. Petr Heinzel

Astronomical Institute, Czech Academy of Sciences

v četrtek, 11. junija 2015, ob 14. uri v predavalnici F2 (Jadranska 19, Ljubljana)

Solar coronagraphs are special telescopes designed to observe the corona and various coronal structures (e.g. prominences) out of solar eclipses. Most efficient are the space coronagraphs, where the observations are not influenced by the Earth atmosphere. I will briefly review previous coronagraphic missions of ESA (European Space Agency) and show some new results obtained with one of them, namely UVCS on board of the SOHO satellite. These results concern large solar eruptions called Coronal Mass Ejections (CME) and their spectroscopic diagnostics. ESA is currently developing two new space coronagraphs, METIS for Solar Orbiter mission (launch 2018) and ASPIICS for Proba-3 mission (launch 2018). The latter one is particularly interesting because it will simulate, for the first time, a total solar eclipse from space.

Finally, I will also mention how CME’s can be detected on other active stars, with their potential influence on exoplanets.

Planetarna meglica Sh2-71 in njena nenavadna centralna zvezda

Teo Močnik

University of Keele, UK

v ponedeljek, 8. junija 2015, ob 13. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Planetarne meglice, ene najbolj fotogeničnih objektov na nočnem nebu, obsegajo pravo paleto različnih oblik in velika večina jih ni sferično simetričnih. Kaj povzroča to nesimetrijo v izvrženem ioniziranem materialu umirajoče zvezde je še danes uganka. Navidezna centralna zvezda bipolarne planetarne meglice Sh2-71 je ena redkih kandidatinj, ki se uvršča na seznam tesnih centralnih dvojnic in tako predstavlja poligon za razvijanje teorij o nastanku nesimetričnih oblik planetarnih meglic. Najnovejša fotometrična in spektroskopska opazovanja razkrivajo presenetljivo spremenljivost in nakazujejo, da je centralna zvezda tesna dvojnica Be + sdO s precesirajočim Be diskom. Tak sistem centralne zvezde je sicer potrebno obravnavati kot nedokončen, a izmed vseh doslej obravnavanih modelov najbolje pojasni opazovane spektrofotometrične lastnosti zvezde in obenem ponudi razlago za bipolarno obliko planetarne meglice.

Modeliranje šuma v astronomskih podatkih

dr. Gal Matijevič

Leibniz-Institut für Astrophysik Potsdam

v ponedeljek, 25. maja 2015, ob 13. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Večina informacij, ki jih dobimo o vesolju onkraj Osončja, pride do nas prek svetlobe. Vsak astronomski posnetek vsebuje poleg signala, ki nas zanima, tudi šum. Del celotnega prispevka šuma je posledica končnega števila fotonov, ki zadanejo detektor in ga lahko modeliramo kot Poissonski proces. Toda skoraj nikoli to ni edini vir šuma.

V obdelanih posnetkih lahko ponavadi zaznamo druge vire sistematičnih in intrinzinčnih variacij, ki so posledica optike, detektojev, obdelave ali pa procesov, ki se odvijajo na astronomskem objektu in jih ne znamo ali ne želimo modelirati. V primeru, da celoten šum npr. v spektru zvezde še vedno obravnavamo, kot da je normalno porazdeljen, lahko zaidemo v težave.

V predavanju si bomo ogledali nekaj orodij, s katerimi lahko efektivno modeliramo celoten šum. Pokazali bomo, da je tak pristop ključen in da so v nasprotnem primeru naše rešitve sistematično premaknjene in navidezno bolj natančne.

The nature of GeV Galactic Center excess emission: options and their testability

dr. Gabrijela Zaharijaš

University of Nova Gorica

v ponedeljek, 25. maja 2015, ob 13. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Several groups have recently claimed evidence for an unaccounted gamma-ray excess over the diffuse backgrounds at few GeV in the Fermi-LAT data in a region around the Galactic Center, consistent with putative signals of long sought for WIMP dark matter particles. However, several conventional astrophysical explanation of this emission appear viable and need to be understood and accounted for, before any robust inference can be made about dark matter signals. In particular, we show that the main features of this excess can be reproduced if they originate in the inverse Compton emission from high-energy electrons injected in a burst event of ~ 10^52-10^53[/latex] erg roughly O( 10^6) years ago and we discuss the testability of this prediction. Another convincing astrophysical candidate for this emission is an unresolved population of MSP. There have been several works studying this possibility and reaching in some cases opposite conclusions. Here we reassess these claims by adopting a phenomenological approach and make clear predictions for the next generation experiments, which due to their improved angular resolution at few GeV should be able to test the point source origin of the excess.

t-SNE: upodobitev N-D astronomskih podatkov z znižanjem dimenzionalnosti

Gregor Traven

Astronomska skupina, Fakulteta za matematiko in fiziko, Univerza v Ljubljani

v ponedeljek, 18. maja 2015, ob 13. uri v predavalnici F4 (Jadranska 19, Ljubljana)

Kako ocenjujemo podobnost? V vsakdanjem življenju se naši možgani spopadajo z ogromnimi količinami vhodnih podatkov, kot so npr. barve posameznih točk našega vidnega polja ter spekter zvoka iz okolice. Te informacije lahko po eni strani predstavimo s točkami v nekem mnogodimenzionalnem vektorskem prostoru, ali pa v bolj kompaktni obliki: rdeča vrtnica, enajstkraka zvezda, ton Cis.

Prav tako se v astronomiji ob vse večjih količinah zajetih podatkov soočamo s težavo njihove vizualizacije oziroma uporabne predstavitve. Tipično je minimalno število parametrov, ki nek proučevani objekt razlikujejo od drugega, precej manjše kot dimenzionalnost zajetih podatkov. V najbolj preprostem opisu lahko npr. spektre zvezd med seboj ločimo glede na temperaturo, gravitacijo in vsebnost kovin. Realno pa spekter zvezde ponavadi sestavlja tisoče ali desettisoče podatkovnih točk.

V predstavitvi si bomo ogledali novo metodo t-SNE, s katero je mogoče skrite zakonitosti v podatkih odkrivati preprosto in učinkovito. Pokazali bomo, kako lahko z zniževanjem dimenzionalnosti uspešno upodobimo tako lokalno kot globalno strukturo podatkovnih točk iz različnih intrinzičnih mnogoterosti. Rezultate t-SNE bomo primerjali tudi s projekcijami konkurenčnih metod na raznovrstnih podatkovnih zbirkah.

Shedding light on the dark Universe with Gamma-Ray Bursts

Ponedeljkov fizikalni kolokvij

dr. Lorenzo Amati

INAF – Istituto di Astrofisica Spaziale e Fisica cosmica, Bologna, Italija

v ponedeljek, 11. maja 2015, ob 16:15 v predavalnici F1 (Jadranska 19, Ljubljana)

[POZOR, SPREMEMBA URE IN PREDAVALNICE]

Gamma-Ray Bursts are the most luminous and remote phenomena in the Universe, with isotropic-equivalent radiated energies up to more than 10^{54} erg and a redshift distribution extending to at least z = 9-10. Thus, they are in principle very powerful tools for cosmology. I summarize the status and perspectives of the research activities aimed at using GRBs to investigate the expansion rate and geometry of the Universe, thus getting clues to “dark energy” properties and evolution, and to explore the early Universe at the end of the “dark ages” (reionization, first stars, star formation rate and metallicity evolution in the first billion of years).

Galaxy Clusters as Tracers of Cosmic Evolution: the Role of Simulations

prof. Stefano Borgani

Univerza v Trstu in INAF – Tržaški astronomski observatorij

v ponedeljek, 13. aprila 2015, ob 13. uri v predavalnici F4 (Jadranska 19, Ljubljana)

I will first briefly review the application of galaxy clusters as tools to trace cosmic evolution and to constrain cosmological models. I will then discuss the recent advances in the this field, as driven by the increasing quality of observational data, and by the much improved description of clusters through detailed numerical simulations.

After discussing successes and criticalities of these simulations, I will present results from their analysis, aimed at calibrating clusters as precision tools for cosmology. In this context, I will discuss (a) possible biases that affect mass estimates based on X-ray and weak lensing data; (b) effects of baryons on the calibration of the halo mass function; (c) robustness of the baryons mass fraction for cosmological tests. I will finally discuss the perspectives for precision cosmology with galaxy clusters offered by the future generation of large multi-wavelegth surveys.

Radijski blišči pri izbruhih sevanja gama

dr. Drejc Kopač

Univerza John Moores v Liverpoolu

v četrtek, 2. aprila 2015, ob 13. uri v predavalnici F2 (Jadranska 19, Ljubljana)

Standardni model zasijev napoveduje prisotnost radijskih bliščev v zgodnjih (<1 dan po izbruhu) svetlobnih krivuljah, kot posledico povratnega udarnega vala. Predstavil bom posodobljen model in rezultate.

Difuzne medzvezdne črte

Janez Kos

FMF, Univerza v Ljubljani

v torek, 2. decembra 2014, ob 12. uri v predavalnici MFP (Peterlinov paviljon, Jadranska 26, Ljubljana)

Z razvojem velikih spektroskopskih pregledov neba se je področje raziskovanja difuznih medzvezdnih črt premaknilo iz natančnih opazovanj redkih vročih zvezd v naši galaksiji proti statističnim študijam masovnih zbirk spektrov, ki jih pregledi neba zagotavljajo. To korenito spremeni metode raziskovanja in predvsem rezultate, ki jih lahko iz novih načinov opazovanja pridobimo. Predstavil bom difuzne medzvezdne črte, množico “skrivnostnih” absorpcijskih črt, ki se pojavljajo v vidnem in bližnjem IR delu spektra pordečenih zvezd, in zadnja opazovanja ter rezultate. Predvsem pa se bom posvetil prihodnosti, ki jo za to področje prinašajo sodobni spektroskopski pregledi neba in možnemu nadaljevanju in razširjenju razikovanja difuznih medzvezdnih črt.

Okolja izbruhov sevanja gama

dr. Jure Japelj

FMF, Univerza v Ljubljani

v torek, 25. novembra 2014, ob 12. uri v predavalnici MFP (Peterlinov paviljon, Jadranska 26, Ljubljana)

Izbruhi sevanja gama predstavljajo zadnji izdihljaj masivnih, hitro vrtečih se zvezd, ki izvirajo na kozmoloških razdaljah. Začetnemu izbruhu običajno sledi svetel zasij v daljših valovnih dolžinah, v katerem so vtisnjene lastnosti galaksije gostiteljice izbruha, kot so zastopanost in značilnost prašnih delcev, prisotnost kovin, galaktično hitrostno polje, itd. Zasiji nam torej omogočajo podroben vpogled v razmere, kakršne so vladale v galaksijah mladega Vesolja. V predavanju bomo izpostavili nekatere vznemirljive rezultate s področja spektroskopskega opazovanja optičnih zasijev, pri čemer bodo še posebej izpostavljena opazovanja z instrumentom X-shooter, montiranim na Zelo velikem teleskopu (observatorij Paranal).

Gamma-ray bursts as probes of galaxy evolution

Dr. Daniele Malesani

Dark Cosmology Centre (DARK), Niels Bohr Institute, University of Copenhagen

v ponedeljek, 12. maja 2014, ob 13. uri v predavalnici F-4

Gamma-ray bursts are powerful explosions at high redshift which probe star formation in a wide set of cosmic environments. I will present our program to extract information from GRB afterglow spectroscopy, to probe the content of gas, metals, dust, and molecules in star forming galaxies across the Universe.

LSST: an era of big data in astronomy

Dr. Emmanuel Gangler

Université de Lyon 1, France

CNRS/IN2P3, Institut de Physique Nucléaire de Lyon

v ponedeljek, 5. maja 2014, ob 13. uri v predavalnici F-4

With the advent of the Large Synoptic Survey Telescope which shall take its first light in 2020, the data flow in optical astronomy will be put to an unprecedented rate. The 3.2 Gpixel-enabled camera will scan all the visible sky in but 3 nights, repeating over the measurements during 10 years, opening up an extensive time domain view of the field. Before concentrating on the emerging new field of astroinformatics which will address the data-related issues both from archive access perspective and new data analysis prospects, I’ll present the LSST project as a whole, and I will briefly review some of the numerous astrophysical topics that will benefit from this massive data set.

The Galactic habitable zone of the Milky Way and M31 from chemical evolution models with gas radial flows

dr. Emanuele Spitoni

Department of Physics, University of Trieste

v četrtek, 24. aprila 2014, ob 10. uri v predavalnici F5

The galactic habitable zone is defined as the region with sufficient abundance of heavy elements to form planetary systems in which Earth-like planets could be born and might be capable of sustaining life, after surviving to close supernova explosion events. Galactic chemical evolution models can be useful for studying the galactic habitable zones in different systems.

We apply detailed chemical evolution models including radial gas flows to find the galactic habitable zones in our Galaxy and M31. We compare the results to the relative galactic habitable zones found with “classical” models, where no gas inflows were included. For both the Milky Way and Andromeda, the main effect of the gas radial inflows is to enhance the number of stars hosting a habitable planet with respect to the “classical” model results, in the region of maximum probability for this occurrence.

These results are obtained by taking into account the supernova destruction processes. In particular, we find that in the Milky Way the maximum number of stars hosting habitable planets lies at 8 kpc from the Galactic center, and the model with radial flows predicts a number which is 38% larger than that predicted by the classical model.

Vloga trojnih sistemov pri določanju osnovnih parametrov zvezd

Dr. Andrej Prša

Villanova University

v ponedeljek, 31. marca 2014, ob 13. uri v predavalnici F4

Prekrivalne dvojne zvezde že od nekdaj veljajo za temelj zvezdne astrofizike, saj z oprijemljivimi metodami lahko določimo mase, radije, izseve in temperature posameznih zvezd v dvojnih sistemih z natančnostjo do 1-3%. Odkar smo deležni podatkov Nasine vesoljske misije Kepler, pa se dvojnicam pridružujejo še večkratni sistemi. Pri predavanju bom predstavil nekaj trojnih sistemov in rezultatov, ki sledijo iz fotodinamične analize, kjer natančnost v posameznih parametrih presega 0.2%.

Hearbeat stars and the ringing of tidal pulsations

Kelly Hambleton

University of Central Lancashire

v četrtek, 27. marca 2014, ob 10. uri v predavalnici F5

With the advent of high precision photometry from satellites such as Kepler and CoRoT, a whole new layer of interesting and astounding astronomical objects has been revealed: heartbeat stars are a prime example of such objects. Heartbeat stars are eccentric ellipsoidal variables that undergo strong tidal interactions at the time of closest approach, when the stars are almost in contact. These interactions cause a significant variation in the surface area of the stars and are observed in the form of a tidal pulse. A subset of these objects ~20% show prominent tidally induced pulsations. We now have a fully functional code that models binary star features (using PHOEBE) and stellar pulsations simultaneously, enabling a complete and accurate heartbeat model to be determined. In this talk we show the results of our new code, which uses Markov chain Monte Carlo techniques, to generate a full set of stellar and orbital parameters. We further highlight some of the interesting features of selected heartbeat stars, including resonant pulsations, frequency modulation, solar like oscillations and apsidal motion.

Observations of Short Gamma-Ray Bursts and future prospects

Prof. Paul O’Brien

University of Leicester

v ponedeljek, 24. marca 2014, ob 13h v F4

Long duration GRBs are thought to arise from the core-collapse of a rapidly-rotating massive star while short-duration GRBs are thought to originate from the merger of compact binary systems. I will summarise recent multi-wavelength observations of short GRBs which have complex light curves which are hard to explain with black holes. An alternative central engine is a magnetar, a massive, rapidly-rotating, highly magnetised neutron star. While consistent with the data, the existence of magnetars and the large derived magnetic fields could pose a major problem for merger models. Future gravitational wave observations may allow us to distinguish between black hole and magnetar models. I will also discuss the current status of GRB finders and the options for future space-based, high-energy missions to monitor the transient universe.

Zgodnja optična emisija pri izbruhih sevanja gama

dr. Drejc Kopač

Fakulteta za matematiko in fiziko, Univerza v Ljubljani

v ponedeljek, 3. marca 2014, ob 13h v F4

Emisija v prvih ~1000 sekundah po izbruhu predstavlja pomembno orodje za razumevanje fizike izbruhov sevanja gama. Opazovanje tako zgodnje emisije preko širokega območja valovnih dolžin nam razkriva mehanizme, ki so odgovorni za nastanek in naravo izbruhov ter nam pomaga potrditi oziroma ovreči različne fizikalne modele izbruhov.

V astrodebati bom predstavil podrobno študijo vzorca izbruhov, pri katerih je bila optična emisija detektirana že med glavno gama emisijo. Z modeliranjem opazovanih svetlobnih krivulj in energijskih spektrov ter s primerjavo s preprostim fizikalnim modelom izbruhov smo pokazali, da lahko v nekaterih primerih zgodnjo optično in glavno gama emisijo povzročijo enaki fizikalni procesi.

Predstavil bom probleme pri modeliranju zgodnjih svetlobnih krivulj in določanju izvora zgodnje optične emisije, ki so pogosto povezani s slabo časovno resolucijo opazovanj ter s predolgim odzivnim časom robotskih teleskopov. Na koncu se bom na kratko posvetil še zgodnji optični polarimetriji, ki lahko predstavlja pomembno orodje za razumevanje zgodnje optične emisije.

Chemical evolution of galaxies: a problem of astroarchaeology

Prof. Francesca Matteucci

Department of Physics, University of Trieste

and INAF – National Institute for Astrophysics (INAF – OATs)

v ponedeljek, 24. februarja 2014, ob 13h v F4

During the Big Bang only light elements (H, D, He, L) were formed whereas all the other elements have been formed inside stars. Chemical evolution of galaxies studies how the majority of chemical elements evolved and distributed in galaxies. To compute chemical evolution we need to assume an history of star formation for each galaxy and take into account possible gas flows (in and out). Stars restore the new chemical elements formed in their interiors at their deaths, which can be quiet or violent (supernovae). Therefore, we need to know the stellar yields, namely the amount of mass in the form of various elements that stars restore into the interstellar medium (ISM) when they die.

I will show how to compute models of chemical evolution for galaxies of different morphological type and the results obtained from these models will be compared to observations. In particular, the comparison between the observed abundances and abundance ratios of the more common elements from carbon to iron and beyond (C, O, Ne, Mg, Si, S, Ca, Fe, s- and r-process elements) to the predicted ones, allows us to impose important constraints on the mechanisms of formation of galaxies as well as on stellar nucleosynthesis. I will show some examples involving galaxies of different type, including the Milky Way, to illustrate how astroarchaeology can shed light on the past history of galaxies and on their early evolutionary phases.

Searching for the dark matter in the Universe

Prof. Francesc Ferrer

Washington University in St. Louis, USA

v sredo, 18. decembra 2013, ob 12h v F3

Numerous experimental efforts to unveil the fundamental nature of the dark matter are underway, and several tantalizing observations have been linked to its effects in recent years. We will review the properties of several candidates that appear in extensions of the Standard Model of Particle Physics, and the different strategies to detect them. Eighty years after the existence of vast amounts of dark matter was first postulated, its non-gravitational interactions might be soon brought up to light.

Cosmological Lithium Problem: A Different Approach

Dr. Tijana Prodanović

University of Novi Sad, Serbia

v ponedeljek, 25. novembra 2013, ob 13h v F5

A discrepancy between primordial lithium abundance predicted by the Big Bang Nucleosynthesis theory, and measurements of pre-galactic lithium abundance in low metallicity systems, has been, for mode than a decade, known as the lithium problem. While measured abundances of other primordial elements are in agreement with their predicted values, lithium in low-metallicity halo stars has been observed to be a factor of 2-4 lower than its expected primordial abundance. No mechanism has been successful in destroying lithium to the observed level so far. Furthermore, the existing lithium problem can become even more severe if any additional pre-galactic source of lithium is present. Possible contaminants could be in the form of cosmic rays that arise during the process of large scale structure formation, or during tidal interactions between galaxies. Gamma-ray observations of the extragalactic gamma-ray background and of galaxy clusters can be used to constrain possible structure-formation cosmic rays and their additional lithium production. On the other hand, measurements of lithium abundance in systems that have recently experienced close fly-bys with neighboring galaxies, such as the Small Magellanic Cloud, can be used to constrain lithium production by tidal cosmic-ray population. Moreover, since lithium has recently been observed in the SMC as its first gas-phase measurement in a low-metallicity system, in order to test the origin of the lithium problem, the importance of determining if lithium was also tidally produced in such system is even greater.

ISON Comet & Iron Tail

Dr. Marco Fulle

INAF – OATs, Italy

v torek, 19. novembra 2013, ob 12h v F3

On November 28, 2013, Comet ISON C/2012S1 will pass its perihelion at a distance of about 10^6 km (less than the Sun’s diameter) from Sun’s surface. In the following week, it is expected to be one of the brightest comets of past decades, possibly brighter than Comet McNaught C/2006P1 or even Comet Ikeya-Seki C/1965S1, the brightest comets of the past century with a perihelion closer than Mercury’s Sun distance. After an introduction to comets, I will explain some of the peculiar phenomena that comets show at these close distances from our star, which allow us to glimpse the metal composition of comet nuclei and the chemistry occurring at high temperature on the surface of dust grains ejected by the comet nucleus.

What Can OPTICON Do for You?

Dr. John K. Davies

OPTICON Project Scientist, UK Astronomy Technology Centre, Edinburgh

v torek, 5. novembra 2013, ob 12h v F3

OPTICON (www.astro-opticon.org) is an integrating activity for optical astronomy which is funded by the EC to the tune of 8.5 million Euro over 4 years (2013-16). I will outline the exisiting OPTICON programme in technology research, community building and its contribution to planing for the next generation of large facilities (E-ELT and VLTI). I will also describe how new communities can gain access to state of the art 2-4m telescopes via the OPTICON Trans-National Access programme and the steps OPTICON is taking to make this process easier.

Spectroscopically Resolved Orbits of Supermassive Binary Black Holes

Dr. Edi Bon

Astronomical Observatory, Belgrade

v torek, 29. oktobra 2013, ob 12h v F3

Active galactic nuclei (AGN) are the most luminous objects in the Universe. There are many scenarios that can explain how AGN are triggered. One of the most intriguing involves the existence of a supermassive binary black hole system in their cores. AGN monitoring spectra can reveal such systems by analysing their emission line shapes and continuum flux variation.
Recently, we discovered the first orbit of sub-parsec supermassive black hole, using very long monitoring campaign spectra and a method typically used for spectroscopic binary stars. We obtained radial velocity curves from which we calculated orbital elements and made estimates about black hole masses. Given the large observational effort needed to reveal this spectroscopically resolved binary orbital motion, we suggest that many such systems may exist in similar objects even if they are hard to find. Detecting more of them will provide us with insight into the supermassive black hole mass growth process.

Solar and Stellar Flares: Observations, Modeling and Synergies

Prof. Petr Heinzel

Astronomical Institute, Academy of Sciences

Ondrejov, Czech Republic

v petek, 11. oktobra 2013, ob 12h v F4

I will briefly review the current status of observations of chromospheric flares on the Sun and other cool stars, with a focus on their spectral properties. Modeling the spectra and light curves represents a challenge for our understanding of the underlying physical processes of the energy transport and dissipation. I will discuss different scenarios of the chromospheric-type emission during flares and point to various synergies between solar and stellar cases. I will also show recent SDO/EVE spectra of the Sun-as-a-star, which can provide important links to stellar flare studies.

Trivialno paralelizabilno vzorčenje

Dr. Anže Slosar

Brookhaven National Laboratory, Upton, NY

v ponedeljek, 30. septembra 2013, ob 10h v F3

Monte Carlo markove verige (MCMC) so standardna metoda za vzorčenje kompleksnih več-dimenzionalnih (N=5-100) verjetnostih porazdelitev v kozmologiji. Kljub temu, da je metoda izredno uspešna, ni razširljiva na računalnike z nekaj tisoč jedri. Predstavil bom pregled problema, teorijo markovih verig in opisal novo metodo, ki obeta veliko boljšo razširljivost in hitrejšo konvergenco. To je delo v teku.

Studying Gamma Ray Bursts from a new perspective

Dr. Giancarlo Ghirlanda

INAF – Brera Astronomical Observatory, Italy

v ponedeljek, 3. junija 2013, ob 13h v F4

I will summarize the current observational status of the prompt emission of Gamma Ray Bursts and discuss some related issues: thermal vs non-thermal components, the nature of the high energy [GeV] component and the “color”-luminosity correlations.

A step forward in understanding the physics of Gamma Ray Bursts is to consider their comoving frame properties. I will show that accounting for the relativistic beaming, GRBs appear to have a nearly “universal” comoving-frame energy/luminosity. This offers a unifying interpretation of the “color”-luminosity relations if there is a link between the geometry and the dynamics in GRB outflows.

Through the results of a population synthesis code the expected dynamical and geometrical properties of GRBs as a population are derived and predictions for the detection rate of GRBs at long wavelengths by forthcoming radio facilities will be presented.

Active Galactic Nuclei in the Fermi and Cherenkov Era

Prof. Massimo Persic

INAF – Astronomical Observatory of Trieste, Italy

v ponedeljek, 13. maja 2013, od 10h do 13h v F6

Predavanje bo izvedeno v sklopu predmeta Astrofizika zvezd in Galaksije za študente na 2. bolonjski stopnji.

A review of the fundamental properties of AGN (sources of energy, relativistic jets, beaming, radiative processes) will lead us to critically evaluate recent and ongoing advancements afforded by the current generation of orbiting and ground-based gamma-ray telescopes. Special attention will be devoted to studying those AGN whose central engine shows up at its most naked – blazars.

Probing the Dark Universe with the Cosmic Growth

Prof. Stefano Borgani

Department of Physics, University of Trieste and INAF – Astronomical Observatory of Trieste, Italy

POZOR, Astrodebata ODPADE

I will discuss the role that cosmological tests based on measurements of the growth rate of density perturbations have in constraining the nature of dark matter and dark energy, as well as the nature of gravity. After reviewing the basic concepts on the evolution of density perturbations, I will highlight how such predictions are altered by modifying gravity. Furthermore I will discuss how neutrino masses can be inferred from cosmological observations, through the effect that massive neutrinos have on the growth rate of perturbations. Galaxy clustering, weak lensing and clusters of galaxies will be discussed as tracers of growth of density perturbations.

Finally I will discuss the role that future large galaxy surveys, like that to be provided by the Euclid satellite, will have in shedding light on the dark sector of the Universe.

The Golden era of GeV astrophysics

dr. Francesco Longo

Department of Physics, University of Trieste and INFN Trieste

ponedeljek, 25. 3. 2013 ob 13h v F4

The contemporaneous observations of the high energy sky (>100 MeV) provided by the AGILE and the Fermi satellites, launched in 2007 and 2008 respectively, are providing the unique opportunity to study the variable gamma ray sky with unprecedented coverage and sensitivity.

In this seminar the main results in the Galactic and the Extragalactic sky by these two missions will be presented and discussed.

Past, present and future of the Intergalactic Medium Cosmology (a European perspective)

prof. dr. Stefano Cristiani

INAF – OATS, Italy

torek, 19. 3. 2013 ob 13h v F3

The history of the universe during and soon after the Dark Age is recorded in the all-pervading intergalactic medium (IGM), which is believed to contain most of the ordinary baryonic material resulting out of the big bang. Throughout the epoch of structure formation, the IGM became clumpy and acquired peculiar motions under the influence of gravity and acted as a reservoir for the gas that gets accreted, cools and forms stars within galaxies and as a sink for the metal-enriched material, energy and radiation which they eject.

Along the line of sight to a distant source – a quasar, a gamma-ray burst, a galaxy – every parcel of IGM gas selectively absorbs certain wavelengths of light due to the presence of the various chemical elements in it. Through the analysis of these absorption lines we can study the spatial distributions, motions, chemical enrichment, and ionization histories of gaseous structures from redshift seven and beyond until the present. From few details of little apparent significance, it is possible to deduce a surprising number of important conclusions about our Universe, especially when we link the information provided by absorption lines with the complementary information derived from the evolutionary properties of luminous galactic structures. The study of the IGM has enormously developed in the last 50 years, and so the contribution of Europe to it, overcoming the initial handicap of collecting power, detectors and tradition.

Prehodni pojavi v Sončevem vetru

dr. Primož Kajdič

Institut de Recherche en Astrophysique et Planétologie (Toulouse, Francija)

ponedeljek, 26. 2. 2013 ob 14h v F5

Tekom predavanja bom opisal pojave, ki so posledica interakcije Sončevega vetra z udarnimi valovi v njem. V grobem lahko udarne valove v Osončju razdelimo na dve skupini: v prvi so planetarni udarni valovi, ki nastanejo zaradi interakcije med Sončevim vetrom ter planeti oziroma njihovimi magnetosferami, v drugi skupini pa so medplanetarni (MP) udarni valovi. Slednji so bodisi posledica hitrih struktur v MP prostoru, kot so medplanetarni izbruhi koronalne mase, bodisi nastanejo z interakcijo med hitrim ter počasnim Sončevim vetrom.

Za vse omenjene udarne valove je značilna odsotnost binarnih trkov med delci (ioni, elektroni), zato pomemben mehanizem za disipacijo kinetične energije sončeve plazme ob prehodu skozi tak udarni predstavljajo interakcije med delci ter magnetnimi polji. Ko je magnetosonično Mahovo število udarnega vala večje od neke mejne vrednosti, je eden od načinov disipacije ta, da udarni val del delcev v prihajajoči plazmi reflektira nazaj v območje pred njim. Če so izpolnjeni določeni pogoji, lahko interakcija teh povratnih delcev z delci v prihajajočem Sončevem vetru močno perturbira območje pred udarnim valom, kar nato privede do cele palete raznih prehodnih struktur v tem območju.

V zadnjih desetletjih smo z raznimi misijami, kot so npr. Stereo, ACE, Wind, Cluster in Themis, dobili jasnejši vpogled v dogajanje v območjih pred udarnimi valovi, kar nam je omogočilo primerjavo teoretičnih izračunov s podatki in-situ.

Cosmic signatures of the early Universe

dr. Umberto Maio

INAF – OATS, Italy

ponedeljek, 18. 2. 2013 ob 13h v F4

Results from high-resolution N-body, hydro, chemistry simulations, including molecule creation, star formation, stellar evolution, metal spreading, and feedback effects will be presented.

Early structure formation and the transition from the primordial population III to the standard population II-I star formation regime will be discussed, with particular emphasis on the implications for high-redshift metal pollution, luminosity functions, (long) gamma-ray burst rate from different stellar populations, and the properties of the corresponding hosting galaxies.

The resulting scenarios in alternative (non-Gaussian) cosmologies will be addressed, as well.

GRB Jet Breaks and Energetics in the Swift/Chandra era

Ponedeljkov fizikalni kolokvij

IZJEMOMA v petek 21. decembra 2012, ob 11:15 v predavalnici F1

Prof. David N. Burrows, Department of Astronomy and Astrophysics, Penn State University, State College, PA, USA

Gamma-Ray Bursts emit tremendous amounts of energy on timescales ranging from less than a second to hundreds of seconds. We have strong evidence that they must be highly beamed, which means that our knowledge of their true energetics relies on understanding the beaming angles of the relativistic jets. The jet opening angles can in principle be measured through the characteristic “jet break” in the GRB light curves. Work a decade ago suggested that these jet breaks occur on typical timescales of a few days. The Swift satellite, which produces detailed, relatively uniform light curves for GRBs that often extend to a few weeks after the explosion, was expected to measure jet breaks on a regular basis, providing good determinations of jet opening angles. To our surprise, this has not been the case. Swift has found much more complex light curves than anticipated on the basis of earlier data, but cases of clear jet breaks are present in only a few percent of the more than 730 GRBs observed to date by Swift. Theoretical work over the past several years has suggested various explanations for the lack of clear jet break signatures in the Swift X-ray light curves.

Recent GRB jet models are now providing a good framework for fitting light curves to detailed hydro/radiation codes that may allow us to finally begin measuring the jet properties for a large class of GRBs. We have begun to apply these models to data sets that feature good X-ray light curves from Swift XRT and Chandra ACIS observations, and I will discuss preliminary results from that work.

Galactic Archaeology in Light of Migrant Stars

Dr. Rok Roškar

Institute for Theoretical Physics, University of Zürich

petek, 21. 12. 2012, ob 10h v F3

The disks of spiral galaxies build up through a variety of processes. The demographics of stars, their kinematic and chemical properties, that we observe today represent a historic record of these past events.

Recently, it has been recognized that stars can efficiently “surf” on the crests of spiral waves throughout the disk, meaning that their present-day locations may have nothing to do with their birth environment. This radial mixing makes the disk a much more diverse place and galactic archaeology much more difficult.

I will discuss the recent advances in our understanding of this process, its implications ranging from astrobiology to extragalactic disks, and place it in the context of cosmological galaxy formation.