DIAS Astronomy & Astrophysics Seminars
Friday, 17.5.2013, 11:00, 31 Fitzwilliam Place
Ievgen Vovk
University of Geneva
Blazars as gamma-ray emitters and probes for the intergalactic medium
Abstract:
I will review the current state of blazar observations in the high
energy gamma-ray domain. I will particularly delve into the question of
the location of the gamma-ray emission site location, reviewing the
recent claims of the detection of absorption features in the blazar
spectra, related to the interaction of the gamma-ray photons with the
background radiation of the Broad Line Region. I will also present our
recent analysis of the variability of the gamma-ray emitting blazars,
giving a hint for the emission site to be located in the vicinity of the
central supermassive black hole.
In the second half of my talk I will discuss, how blazar gamma-ray
observations can be used to infer some properties of the intergalactic
medium. As very high energy (VHE) gamma rays from distant blazars
interact with lower energy photons, producing electron-positron pairs,
they can be used to probe optical and infrared light background light,
present in the intergalactic space. This Extragalactic Background Light
is the integrated light of all the galaxies in the Universe, and its
investigation provides an important handle on the star formation history
in galaxies at different redshifts. I will review our recently prepared
catalogue of VHE blazars, that can be used for this kind of studies. I
will also discuss, how gamma-ray emitting blazars can be used to get
information about the extremely weak magnetic fields, present in the
intergalactic space. I will review recent findings on this matter and
give prospective for currently existing and future gamma-ray telescopes.
TCD/DIAS seminar: Friday, 26.4.2013, 15:00, Schroedinger Lecture Theatre
Alexander Brown
University of Colorado at Boulder
KEPLER Observations of Starspot Evolution, Differential Rotation,
and Flares on Late-type Stars
Abstract:
Starspots are direct tracers of regions of intense magnetic fields
in the photospheres of late-type (F-G-K-M) stars. These magnetic
fields originate from a internal ``dynamo'' process powered by the
interaction of subphotospheric convection and differential rotation.
Observations of the surface distribution of starspots and how that
distribution evolves provide important information on the dynamo
process and the magnetic activity that it generates. The Kepler
satellite is providing spectacular optical photometric light-curves
of unprecedented precision and duration that routinely allow detailed
studies of stellar magnetic activity on late-type stars that were
difficult, if not impossible, to attempt previously. Rotational
modulation due to starspots is commonly seen in the Kepler
light-curves of late-type stars, allowing detailed study of the
surface distribution of their magnetic activity. Kepler
is providing multi-year duration light-curves that allow us to
investigate how activity phenomena -- such as the growth, migration,
and decay of starspots, differential rotation, activity cycles,
and flaring -- operate on single and binary stars with a wide range of
age, mass and convection zone depth.
In this talk I shall present results that demonstrate the powerful
diagnostic capability provided by tracking starspot evolution essentially
continuously for years at a time and speculate on what might be possible
over the planned 8 year life-time of the Kepler mission. Using ground-
and space-based observations we have investigated the populations of
active stars present in the Kepler Field and I shall show how these
different groups of stars reveal themselves in their Kepler light-curves.
To interpret Kepler photometric time-series we have developed new
light-curve inversion codes that fully utilize the quality of the
Kepler data and will be the tools that allow us maximize our understanding
of stellar magnetic activity over the coming years.
Thursday, 18.4.2013, 15:00, 31 Fitzwilliam Place
Malcolm Walmsley
DIAS/Arcetri
The origins of high mass stars
Abstract:
High mass stars are used to trace the star formation rate both in the
Galaxy and in external galaxies. However, our understanding of how they
form is still rather primitive in comparison to current models of the
formation of solar mass stars in the solar neighbourhood. We do know
however that most high mass stars form in clusters or associations
whose mass in the Milky Way can be at least 10000 solar masses and whose
extent is just a few parsec. Thus, to a certain extent, understanding high
mass star formation requires understanding cluster formation.
I will try to summarise what we know about
the origins of massive clusters as well as the connections to the
high star formation rates found in extragalactic systems. I will also
briefly describe some recent results based on galactic surveys aimed
at determining the properties of the forerunners of galactic clusters.
Finally (if time permits), I will describe some very recent results
based on Cycle 0 ALMA data where we studied the disc around a putative
high mass star.
Friday, 12.4.2013, 15:00, 31 Fitzwilliam Place
Aleks Scholz
DIAS
SONYC: The director's cut
Abstract:
SONYC - Substellar Objects in Nearby Young Clusters - is an ultra-deep survey of nearby star forming
regions to provide a census of brown dwarfs and planetary-mass objects (planemos) down to
a limit of 5 Jupiter masses. The project is complete for four regions and has so far identified and
characterised more than 50 previously unknown brown dwarfs, including a few objects with masses below
the Deuterium burning limit (15 Jupiter masses). Based on these new samples, we have been able to
solidify and revise our knowledge about the early evolution of brown dwarfs and their disks. We find
that for every 10 stars formed there are 2.5-5 brown dwarfs and 1 planemo. Thus, brown dwarfs and planemos
likely represent the low-mass tail of the stellar initial mass function. In this talk, I will give
an overview about the project, its motivations, its logistics, and its scientific findings.
Thursday, 11.4.2013, 15:00, 31 Fitzwilliam Place
Claudio Zanni
INAF - Osservatorio Astrofisico di Torino
Magnetic star-disk interaction and the stellar angular momentum
Abstract:
Classical T Tauri stars are characterized by slow rotation periods,
roughly corresponding to 10% of their break-up speed:
a large fraction of the stellar angular momentum has been plausibly
extracted during the embedded phase. Moreover, the
rotation period seems to stay constant during the T Tauri phase, despite
the fact that the protostar is still actively accreting
and contracting: an efficient mechanism of angular momentum removal is
therefore required. Making use of numerical MHD
experiments of the interaction of an accretion disk with the
magnetosphere of the protostar, I will review different processes
that have been proposed to balance the spin-up torque associated with
accretion. In particular, I will illustrate the effects of
an extended star-disk magnetic connection, stellar winds and unsteady
magnetospheric ejections.
Friday, 22.3.2013, 14:00, 31 Fitzwilliam Place
Ruyman Azzollini
DIAS
Near-UV clumps in Disc Galaxies in the range 0<~z<~1
Abstract:
The global Star Formation Rate in galaxies has declined by roughly tenfold since
z~1. The relation between this progressive dearth of newborn stars and the
morphologies, stellar masses, and other relevant properties of the harbouring
galaxies has been the subject of much research in past years. Nonetheless, even
basic knowledge on how this phenomenon relates to the spatial
distribution, and aggregation properties, of the regions where this star-forming
activity takes place within galaxies is quite limited. We aim at characterizing
basic parameters of the star-forming regions within disc galaxies where intense
star-forming activity takes place, and their underlying stellar populations,
during the last ~8 Gyr of cosmic time.
We survey a sample of 31 disc galaxies at z ~0 and 239 in the range 0.5< z
<1.1, in search of near-UV clumps, defined as significant excesses in flux over
the azimuthally-averaged surface brightness profiles of the galaxies. The study
uses publicly available imaging datasets from GALEX, SDSS and GOODS-HST/ACS. We
present a preliminar analysis of the multi-band (near UV to near IR) photometry
of these NUV-clumps, their sizes and radial distribution, and the basic
properties (e.g. SFR and stellar masses) of the underlying stellar populations
in these regions, by comparing of the observed SEDs to simple models. We also
present an interpretation of the results in relation to the evolution of the gas
and stellar content of these galaxies.
Friday, 1.3.2013, 14:00, 31 Fitzwilliam Place
Chris Davis
Liverpool John Moores University
Outflows as tracers of star formation: what high resolution observations
and wide field surveys can teach us
Abstract:
Two broad areas of study that have led to an improved understanding of
the accretion and outflow process in star formation will be discussed:
detailed observations of the central engine (the star-disc system
driving the jet/outflow), and wide-field surveys of star forming regions
on parsec scales. Both can potentially teach us much about the
collimation and acceleration of outflows, how these flows interact with
their surroundings, and about star formation as a whole.
Liverpool Telescope: a brief overview of the Liverpool Telescope,
Europe's premier robotic observatory, will also be given prior to the
above presentation. The LT specializes in delivering high-impact
results in time-domain astrophysics, and is the largest facility of its
kind in the world. In the run-up to LSST operations in the next decade,
support for the LT (and it successor, LT2), is vital if the UK is to
maintain its leadership role in rapid-response and transient astronomy.
Friday, 15.2.2013, 15:00, 31 Fitzwilliam Place
Caroline d'Angelo
Amsterdam University
Examining Exors: A magnetic model for variability in young stars
Abstract:
The T Tauri star EX Lupi is the prototype of the 'EXor' class of T Tauri
stars, which show recurrent, unexplained outbursts on a timescale of
several years. In this talk I will present work on a disk instability
that could explain the outbursts. The instability arises when the
strong magnetic field (~1 kG) of a protostar truncates the surrounding
accretion disk near the co-rotation radius (where the Keplerian
frequency matches the rotation rate of the star). When the disk is
truncated just outside co-rotation, the interaction between the inner
regions of the disk and magnetic field exerts an outward torque on the
disk, allowing a reservoir of mass to build up in the disk's inner
regions. The increased mass in turn increases the viscous torque in
the disk, which opposes the magnetic torque and allows the inner edge
of the disk to push inside the co-rotation radius, whereupon the disk
can accrete freely through the magnetosphere onto the star. Once the
reservoir of mass has been drained, the disk moves back outside the
corotation radius, and the cycle starts again. I will discuss the
mechanism and compare the predicted outburst profiles to detailed
observations of EX Lupi's 2008 outburst as well as more recent work on
new EXors.
Friday, 8.2.2013, 15:00, 31 Fitzwilliam Place
Gaitee Hussain
ESO Garching
The role of magnetic fields in pre-main sequence stars
Abstract:
Strong, kilogauss, magnetic fields are required to explain a range of
observational properties in young, accreting pre-main sequence systems.
I will review the results from our five-year campaign aimed at
characterising the magnetic field properties of young T Tauri stars. The
magnetic field observations have been used to build 3-D models exploring
the role of magnetic fields and the efficiency with which magnetic
fields can channel accretion from circumstellar disks on to the young
star. In addition I will present the first results from a new programme
aimed at investigating the evolution of magnetic fields in intermediate
mass (2-5 Msun) pre-main stars as they become fully radiative.
Friday, 1.2.2013, 15:00, 31 Fitzwilliam Place
Eamon O'Gorman
Trinity College Dublin
Radio Interferometric Studies of Cool Evolved Stellar Mass Outflows
Abstract:
Evolved stars are very important contributors of material to the interstellar medium. However, for
evolved spectral type K through mid-M stars (i.e. red giants and red supergiants) the mechanisms
by which this matter is expelled remain a mystery. I will first discuss the results of our latest
millimeter multi-configuration interferometric study of the massive red supergiant Betelgeuse using
the Combined Array for Research in Millimeter-wave Astronomy (CARMA). I will discuss the
significance of these results along with ongoing and future efforts to understand the complex nature
of Betelgeuse's mass outflow region. I will also discuss our continuing effort to probe the
thermodynamics of two non-dusty red giant stars, Arcturus and Aldebaran and present our multifrequency
Karl G. Jansky Very Large Array (VLA) thermal continuum observations of these two
objects. I will present spectral energy distributions for both stars and compare these with previous
measurements and models. Finally, I will describe our hydrogen ionization code which we are using
along with these new VLA measurements to compute temperature profiles for both star's mass
outflow regions.
Friday, 11.1.2013, 15:00, 31 Fitzwilliam Place
Jan-Uwe Ness
European Space Astronomy Centre (ESAC)
High-resolution X-ray spectra of Super Soft X-ray Sources
Abstract:
Super-Soft X-ray Sources (=SSS) are a small class of X-ray sources characterised by a blackbody-
like spectrum of effective temperature 30-100 eV (several 10^5 K) and luminosities above 10^36 ergs.
Owing to their softness, galactic SSS are more difficult to observe and interpret because of high
galactic extinction and uncertain distances, respectively. While a small number of permanent SSS
is known since the 80s, novae have been predicted to pass through a phase of SSS emission that has
indeed been observed with, e.g., ROSAT, BeppoSAX, or ASCA. Attempts of spectral modeling of
nova SSS spectra ranged from blackbody fits to most refined LTE and non-LTE atmosphere modeling,
but the low resolution of CCD spectra allows no unique constraint of spectral parameters of complex
models. The X-ray grating spectrometers on board XMM-Newton and Chandra allow much more
detailed analysis of SSS spectra and, as always in nature, the truth is much more complicated than
believed. I will first present historic observations and attempts of interpretion, and then show the
grating spectra with the details. A large variety in grating spectra of canonical SSS spectra and those
of novae emerged, and I will show approaches how to find trends and to explain some commonalities.
Spectral modeling is currently not possible, but I will present and discuss some approaches.