Time-Resolved Properties of the White Light Continuum During Stellar Flares/ Seminar Series (4)

Speaker:Adam Kowalski
Affiliation:
Astronomy Department, University of Washington
Title: Time-Resolved Properties of the White Light Continuum During Stellar Flares
Date: 17 February  2011
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:
The primary mode of radiative energy release in stellar flares is in the optical and near-ultraviolet (NUV) continuum. This white light radiation carries a large fraction of the total radiated flare energy. However, radiative hydrodynamic models of stellar flares using a solar flare paradigm and the sparse observations of solar and stellar flare continua are all seemingly in disagreement over the type(s) of emission that contribute to the optical/NUV continuum during flares. We have begun a long-term spectroscopic flare monitoring campaign to fully characterize the  optical/NUV white light continuum emission on short timescales for large and  small flares. To date, our most significant results come from spectroscopic (3350A - 9260A) and photometric (U band) observations during  1.3 hours of the decay phase of a megaflare on the dM4.5e star YZ CMi, where  we have detected multiple continuum components that contribute to the white  light. I will present the continuum and emission line properties of this  flare and initial phenomenological modeling of the flaring atmosphere. I will  also compare the continuum properties to ultra-high speed observations of 
smaller flares

Gravitational wave sources and the (future) use of the La Palma telescopes / Seminar Series (3)

Speaker:Paul Groot
Affiliation:University of Nijmegen, The Netherlands
Title: Gravitational wave sources and the (future) use of the La Palma  telescopes
Date: 9 February  2011
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:
The gravitational wave domain remains the last completely unopened window on the Universe. Ground-based high frequency detectors are already operational, but lack known sources. The low-frequency domain will be opened by the LISA space interferometer. The only known LISA sources are ultracompact white dwarf binaries. In a campaign involving many of the  
La Palma telescopes we are uncovering and characterizing the Galactic
population of these ultracompact binaries, with orbital periods as short
as 5.6 minutes. In the talk I will give an overview of the current state of affairs, our use of the La Palma telescopes, and the (possible) future (combined) use of the telescopes.

Accelerating Universe: Dark Energy vs Modified Gravity / Seminar Series (2)

Speaker:Shant Baghram
Affiliation:Sharif University of Technology
Title: Accelerating Universe: Dark Energy vs Modified Gravity
Date: 21 January 2011
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:

First, I will give a very brief description of the accelerating universe state and its cosmological evidence like SNIa, CMB and the LSS. Then I propose the Cosmological constants (LCDM-model) and its alternative Dark energy (DE) and Modified gravity (MG) models as plausible candidates for describing the accelerating Universe. Some cosmological observations, especially large scale structure probes such as matter power spectrum, the ISW effect and growth index are discussed later, as useful tools to distinguish between viable DE and MG models which are equivalent in predicting the background dynamics of  Universe. Finally, the reconstruction of the dynamics method as a probable way for investigating the problem is introduced, and the future prospects on the issue is discussed. 

Filter Measurements for ACAM / Seminar Series (1)

Speaker: Annemieke Janssen, PhD Student
Affiliation:Rijks Universiteit Groningen & ING student
Title: Filter Measurements for ACAM
Date: 10 December 2010
Place: ING+NOT Astro-seminar- Santa Cruz-La Palma- Spain

Abstract:

During the last three months the Transmitted Wavefront Distortions (TWD)
of almost all 50-mm filters have been measured. These distortions are
important to know since they affect the image quality of ACAM
observations. For other instruments the TWD is less critical since filters
are usually placed in the focal plane, where they only affect the image
quality locally. ACAM is a bit of an exception having the filters placed
near the pupil plane, where wavefront distortions over the whole filter
area affect the image quality.

We started analysing the results, partly with Zemax and partly with
pinhole experiments. In the last case, calibration lamps and a pinhole in
ACAM create a very tiny spot on the CCD. The effect of a filter placed in
the beam can now be compared with the measured TWD. The first results will
be discussed in an half-hour presentation.

Scintillation of light curves and Dark matter

In the last two weeks, in physics department of SUT, in our weekly cosmology seminars we have Frahang Habibi as a speaker. In this two sessions he describes for us a general view of his Ph.D thesis. Farhang is a joint Ph.D. student of sharif university and IAP in Paris.
He is working on dark matter detection in form of hydrogen molecule gas.

Now what is the story?
As we know 23% of our universe is composed of dark matter/means no electromagnetism interaction/but gravitationally is resemble to ordinary baryonic matter.
There are many many candidates for Dark matter starting from super symmetric particles to unseen baryonic matter.
Now what is Farhangs and Mark Moniez/his french side supervisor/ idea ?It is about a cloud of hydrogen molecule distrubeted in clusters which are not radiating because of there internal structure of hydrogen molucule ,but can be a candidate of unseen matter.
In this thesis they introduce a type of a test in order to detect them.
If there is a cloud of hydrogen molecules, they can effect optically on the light coming from stars beyond it.This cloud can scatter and diffract the light coming.So if in an experiment / observation/we can see an effect of scintillation in the light curves of the stars., it shows that there is something there that cause the amplification of light and maybe if the characteristics of our observation seems to our models match, it could be a cloud of hydrogen.

This is the good part of the story.But the bitterly hard part remains,the simulation of this effect and then observation of a known cluster and after that data reduction in order to find the best candidate stars which their light curves shows the effect of scintillation is Farhangs hard task.

Chameleon mechanism

Tuesday 28 of October I gave a small!!! talk in physics department,in series of cosmology weekly seminars.

The general title of talk is the Accelerating universe,Theory and phenomenology and the subtitle is Chameleon scalars mechanism and modified gravity.

The general title is the field which I want to be engaged in my Ph.D.thesis.

And what about the chameleon theories?

The accelerating universe open a new paradigm in cosmology.

There are many many models trying to describe this phenomena,but generally we can divided all the models in 2 general subdivisions.

If we accept the context of general relativity, which means that we know gravity well, we will come to the realization that new physical entity needed to describe the accelerating universe,such as cosmological constant or scalar fields generally named quintessence.

The second main models come to existence from the assumption that the general relativity needed to be modified in long ranges and small curvatures.This models named Modified gravity.

But what is chameleon?

The point is that we know that general relativity works well in solar system and local tests of gravity.So if we change the action of Einstein -Hilbert to have new effects in cosmological scales also we will affect the local tests of gravity, so we need a mechanism! in order to conceal this effect in local tests and disappear it and bring it back to existence in cosmological scales. This mechanism is called Chameleon mechanism after the name of lizard!!!like animal which changes its skin color with environment.So the scalar field equivalent to modified gravity /We know about the equivalence of modified gravity with scalar field theories/

changes its behaviour with respect of the density of environment like Chameleons.

BUT

in theoretical point of view this mechanism needs more investigations to be reliable,I will study it more...

Ads/CFT

It is about a year that in our physics department in Sharif university, each Tuesday held a seminar among the three active theoretical groups of department.

1)Particle physics 2)Cosmology 3)Conformal field theory group

The idea and the motivation of seminars is nice:it is to be connected and aware of the work of each other.Last Tuesday particle group talk about really complex subject.I want to respell the ideas of it in order to have a snapshot of this.

The talk is given by Mohsen Alishahia and is about about Ads/CFT correspondence more vastly Anti de sitter space/Conformal field theory correspondence which is also called after Maldacena the Argentinian physicists MAldacena duality.What is all about?

The theory shows that the string theory in a Anti desitter space plus a sphere or orbifold is physically equivalent to a conformal field theory with out gravity defined in its boundary.

It seems complicated anyhow it is interesting because of presenting a way to solve the quantum gravity theory and also it is inspired by the holographic principle which claim that you can extract the physics of a volume from its boundary.Holographic principle is a a speculative idea about quantum gravity originally proposed by Gerard 't Hooft and improved and promoted by Leonard Susskind.

Finally the lecturer imposed the probability that maybe this correspondence first of all can be used to describe the gravitational effects on inflationary models.

Let me show up some important papers on this genius work.

Reminding you that In about five years, Maldacena's article had 3000 citations and became one of the most important conceptual breakthroughs in theoretical physics of the 1990s, providing stark new insight into both quantum gravity and QCD.

1)Maldacena, The Large N Limit of Superconformal Field Theories and Supergravity, arΧiv:hep-th/9711200.
2)Witten, Anti-de Sitter Space and Holography, arΧiv:hep-th/9802150.
3)Gubser, Klebanov and Polyakov, Gauge Theory Correlators from Non-Critical String Theory, arΧiv:hep-th/9802109.
4)Aharony et al, Large N Field Theories, String Theory and Gravity, arΧiv:hep-th/9905111.
AdS/CFT correspondence on arxiv.org.

MOND vs Dark Matter

A couple of weeks ago, there was a Ph.D. thesis defence in Sharif University of Technology by Hossein Haghi **,***under supervision of Dr. Sohrab Rahvar/He is my supervisor too!:)/, in physics department with the title of MOND* in ... : Modified Newtonian Dynamics.

What is it about?and what is the story of modification...

During last century, in physics history usually we observed new phenomenas in experiments which could not be described by approved and used models and physical laws of the day.So what happened? Immediately the situation is divided in two ways

1)In the same frame of our known laws and models, we defined new physical entities to describe the experiment. or

2)We concluded that we do not know the laws well, and we start to change and modify them.

This scenario was happened several times, as a famous example is the problem of missing mass in beta decay in late 20's and beginning of 30's ,as I mentioned there were two solution 1)The law of conservation of energy is not correct or

2)there is a new mysterious unseen particle that we do not know about it and can not detect in experiment.

The history in this case shows that the second solution, the famous Neutrino predicted by Pauli was the right answer.

Now we have such a situation in galactic scale cosmology too.

The rotation curves of stars rotating in galaxy contradict with the prediction of Newtonian Mechanic, and we get flat rotational curves.

Immediately there is two solutions to the problem

1)Modifying the Newtonian law:Which means we do not understand it well,or there is more complete theory standing there.MOND as an example.

2)There is a new mysterious,only gravitationally interacting particle which named DARK MATTER ,which is responsible for this new missing mass in galaxy.

Finally what I want to mention is about the always existing two perspectives as a solution of the problem.

1)Modifying the laws or 2)Introducing New physical entity, are always teasing physicists minds.

But which one is seems more plausible is not yet well established but it will be fair to mention that the DARK MATTER campaign is so large /because of its success in cosmological model for structure formation models as CDM and there is a huge Hope to find light supersymmetric particles (LSP) in LHC arXiv:0807.2244 as a candidate for this entities as DARK MATTER.

But the story is not happy ended yet .Anyhow let us see what experiments unreveal for us.

*Nice web site about MOND related papers: http://www.astro.umd.edu/~ssm/mond/

**arXiv:astro-ph/0606246

***arXiv:astro-ph/0504171