Talk on Monday: "The Red, Featureless Outer Disks of Nearby Spiral Galaxies" by Dr. Aaron Watkins (University of Oulu, Finland)

Date: Monday 11 September at 11:15
Place: IT 133

Dr. Aaron Watkins will present results from deep, wide-field surface photometry of three nearby (D = 4 – 7 Mpc) spiral galaxies: M94 (NGC 4736), M64 (NGC 4826), and M106 (NGC 4258). The extraordinary depth of this imaging allows for accurate surface photometry and characterization of integrated stellar populations far into the outer disk (~5 – 10 scale lengths).

Combining this deep imaging with archival ultraviolet and 21 cm data, Dr. Aaron Watkins find that each galaxy has a well-defined radius beyond which little star formation occurs and the disk light appears both azimuthally smooth and red in color, implying predominantly old, well-mixed stellar populations. This sharp outer disk demarcation is present despite that each galaxy shows quite different levels of environmental influence.

This, combined with the lack of ongoing star formation, argues that the outer disks of these galaxies were likely formed via some common secular process, such as the outward migration of old inner disk stars via interaction with bars or spiral arms (radial migration). However, the large physical extent of these smooth outer disks – which reach several disk scale lengths beyond each galaxy's visible spiral structure – may present a challenge to the radial migration model given the lack of any nonaxisymmetric forcing at such large radii.

Past seminars

Astronomy Seminar: "What we could learn about Saturn's rings" by Lic. Annabella Mondino (Observatorio Astronómico, Córdoba, Argentina)

Date: Wednesday 30 August at 14:15-15:00
Place: MA 341 (Mathematics corridor seminar room)

Astronomy Seminar: "Pulsar-like white dwarfs in cataclysmic variables." by prof. Nazar Ikhsanov (Pulkovo Observatory, St.Petersburg, Russia)

Date: Tuesday 28 August at 12:15
Place: MA 341

Marsh et al. (Nature, 537, 374, 2016) have recently reported the discovery of a radio-pulsing white dwarf in the cataclysmic variable AR Sco. The period of pulsations which are also seen in the optical and UV is about 117 seconds. High intensity of pulsing radiation and non-thermal character of its spectrum leave little room for doubt that the white dwarf in AR Sco operates as a spin-powered pulsar and, therefore, is in the ejector state. Using approach earlier developed for interpretation of the 33 seconds period pulsar-like white dwarf AE Aqr (Ikhsanov A&A, 338, 521, 1998; Ikhsanov & Biermann A&A, 445, 305, 2006; Ikhsanov & Beskrovnaya Ast. Rep, 56, 595, 2012) I evaluate the surface magnetic field of the white dwarf in AR Sco as 100-300 MG. I argue that this source has originated due to accretion-driven spin-up in a previous epoch during which the magnetic field of the white dwarf had substantially evolved being initially buried by the accreted matter and recovered to its initial value after the spin-up phase had ended. I show that the mechanism of pulsing radio emission in this source is rather puzzling. The large radius of the white dwarf makes general relativity effects at its surface negligible and the magnetic field is too weak for pare creation to occur. Possible alternative mechanisms for particle acceleration and generation of radio emission in the magnetosphere of the white dwarf are briefly discussed.

Astronomy Seminar: "Radiation efficiencies of pulsars. Optical observations." by Dr. Sergey Zharikov (Instituto de Astronomia, UNAM, Mexico)

Date: Tuesday 22 August at 14:15
Place: MA 341

Eighteen pulsars with optical counterparts or with significantly deep upper limits on the optical luminosity are known currently. In this talk, I will present an analysis of efficiencies of conversion of the pulsar spin-down power into the observed non-thermal luminosity for different spectral domains. The sample of pulsars confirms the non-monotonic evolution of the pulsar radiation efficiency in the optical and X-rays. There is a clear evidence of a change in the behavior of the optical and X-ray efficiencies around ~104 years. Both the optical and X-ray efficiencies initially decrease before starting to flatten or increase at larger ages. The timescale of 104 years is comparable to the transition between neutrino and photon cooling stage in neutron stars. The change of the cooling stage probably affects the distribution of relativistic particles in the pulsar magnetosphere, which is reflected in the dependence of the optical/X-ray efficiency on the pulsar age. The slopes of the time evolution of the optical and X-ray efficiencies after 104 years are practically similar and compatible with that of the time evolution of the Radio efficiency. In addition, a short review of new optical observations of pulsars will be presented.

Astronomy Seminar: A detailed view into a jet-driving binary with a circumstellar and circumbinary disk by Dr. Fernando Comerón (ESO, Chile)

Date: Tuesday 14 February at 14:00
Place: SÄ124

Multiplicity is a frequent result of the star formation processes, and dynamical interactions among components happen frequently in the early evolutionary stages of multiple systems, leading to the formation of close binary pairs, hierarchical triples or higher-order systems, and to the ejection of components. It has been proposed that these early dynamical interactions may be responsible for episodes of destabilization of circumstellar disks, causing temporary high mass loss rates that leave an imprint in the Herbig-Haro jets often observed near young stellar objects. Pursuing this idea we have carried out a small survey of very young stellar objects known to drive Herbig-Haro outflows or having displayed FU Orionis-type outbursts in the recent past. This has led to the discovery of the binary character of the driving source of HH 250, a virtually unexplored system showing evidence for gas-depleted circumstellar envelopes around each component and a gas-rich circumbinary disk. The intriguing characteristics of this system as revealed by several instruments at the Very Large Telescope in Chile and by the Sub-Millimeter Array Hawaii, as well as its promising potential for the study of the driving source of jets, will be described

History of Astronomy Seminar: The Road to the Big Bang by Dr. Sébastien Comerón (University of Oulu)

Date: Wednesday 8 February at 18:15-20:00
Place: TELLUS Stage

On 8 February 1917 ALBERT EINSTEIN presented the application of the equations of general relativity to the Universe at the Prussian Academy of Sciences. This marks the beginning of physical cosmology.

The Big Bang theory is the present-day explanation to the origin and the evolution of the Universe. Its development has been one of greatest hits in the history of science during the 20th century. In this talk, Sébastien Comerón will review the facts that, in his opinion, are relevant to the establishment of the Big Bang theory. He will rapidly cover the two millennia that elapsed between Ancient Greece and the consolidation of the Big Bang cosmology. While this review can by no means be complete Sébastien Comerón will try to highlight some brilliant works have gone unnoticed through history.

Origin of Bulge in the Milky Way by Alan Su (University of Lund)

Date: Tuesday 31 January at 9.00-10.00
Place: MA341 (in Mathematics Corridor)

In the context of the Milky Way, the origin of the Bulge has been highly debated in the literature. One method of study is to utilise spectroscopy as a tool to analyse stellar populations, in order to infer and constrain galactic parameters (e.g. SFR/IMF/metallicity gradient). However, due to high extinction, it has only recently been possible to study the innermost parts of the Bulge via near infrared spectroscopy. Currently there is ongoing debate as to the distribution of the metallicities of stars within the region. Alan Su master’s project aims to contribute to the debate by reanalysing data from previous studies. A different methodology was utilised to determine the metallicity to create a homogenous dataset of stars in the inner Galactic Bulge.