Subject: Re: talks From: Véronique Petit Date: Fri, 08 May 2009 23:13:57 -0400 To: David Cohen Hi David, I am sorry to get in touch so late. I was in a laptop-prohibited-during-the-40-or-so-talks conference the last two days and I didn't get time to type my talk draft until just now. I haven't put anything onto slides yet, so I can easily wrap around what you have prepared already. As I will be the first speaker to focus more on wind-field interaction, I will spend a bit of time introducing that. Asif told me he will be mainly focusing on what would influence the cooling. I was planning to discuss the ONC more as a whole, but I probably should spend a slide each on Par and NU to discuss their respective properties. Is there something in particular you would like me to mention ? We haven't agreed about who would talk about the Chandra proposal, so I added that at the end of my talk, but I can remove it if you are talking about it. I am flying to Paris tomorrow at the end of the afternoon, but from Montréal, and I will have to leave early (like 10am) because the bus schedules do not fit quite well. Cheers, V. ---------------------------------------------------------------------------------------------- * Wind-field interaction. ** If a large-scale magnetic field is present, it will try to force the wind to move along its field lines ** On the other hand, the wind will try to force the field lines to move with its radial flow ** There is a duel of force. If the magnetic field is the boos, the wind will flow along the field lines and collide at the magnetic equator. * This wind-field interaction was modelled by Asif's MHD simulations. The simulation showed that : ** the flow will produce strong shocks (as the collision is heads one) that will heat the gas. These shocks will therefore be less energetic than the shocks that usually take place in a normal wind (What typical temperature ???). These shock are produce by collision between streams of winds that flow at slightly different speed, due to the intrinsic instabilities of the line-driving mechanism. ** Unlike the "normal wind shocks" that are happening all the time everywhere in the wind, the magnetic shocks will lead to some dynamical event on an longer time-scale: centrifugal breakouts, reconnection even.. * The heated gas will cool down, emitting X-rays in the process (Asif will talk more about that) ** In non-magnetic winds, the X-rays will be soft (peaking at around X KeV) and stable. Observationally, the X luminosity will be 10-7 time the total luminosity for type B1-allO, and decreasing for later spectral type. ** The magnetic winds will potentially result in X-rays that will be harder, variable (ie flare) and more luminous than those of non-magnetic stars. Depending on the geometry of the sustem, and how the X-rays produced in the magnetosphere are absorbed or occult by the far wind and/or the star itself, it may result in periodical modulation. * Archetype: Theta 1 Ori C ** O-star with a 1kG field. Simulation were done for that star specifically, and it explain very well all the peculiar aspects of this star:
Lumious and hard X-rays, modulated with the same period as the variation in UV wind lines and visible photospheric and emission lines.
X-ray narrow spectral lines (maybe David will talk about that more) Even the slow rotation for a young O-star = magnetic breaking. * On one side: soft and stable X. On the other side: hard, overluminous and variable X. ** Does this conclusion apply in all cases ? Or only 1C ? This is important because we need to understand how field generally affect the atmosphere and wind of a star, hence modifying the observables in all wavelength. If we want to interpret correctly the observation of massive stars, this will be necessary. ** Could X-rays be used to detect magnetic stars ? It would be really useful to identify magnetic stars, and also used to determine the incidence of field in massive stars. Think about it, it is "easier" to take some X-ray observation of a big sample of stars, with large-field camera like XMM or Chandra, than to take spectropolarimetric observations of all these stars. In the best best best case, would we be able to determine some useful information about the magnetic properties of the stars with the X-rays information ? (other than that it exist...) * For my PhD, we decided to test these hypothesis by doing a spectropolarimetric survey of the 8/9 massive stars of the Orion Nebula Cluster. ** We measured the circular polarisation at at least two epoch (except JW660) with ESPaDOnS@CFHT. ** We performed a careful LSD analysis (tests with 1C if I have time to finish them....?) * Surface fields: In the context of a survey, a MDI technique is not really appropriated as it requires too much data over the rotation period (which is unknown in most of our stars anyway). We don't want to waist too much observation time on a non-detection. So we developed a technique based on Bayesian statistic and an approximation to the polarized radiative transfer. * We suppose a simple dipole geometry, and we compare the observed profiles to an interpolated grid of synthetic Stokes V profiles * Numerical tests of Mr. Bayes ? * Result: simple classification of observed stars with X-ray characteristics. ** (-- compute the eta_star) *Important conclusion: Variable, energetic and luminous X-rays are not systematically correlated with the presence of a large-scale magnetic field. * Todo: More detailed study of known magnetic stars to see under which circumstances they will produce unusual X-rays. ** We need better magnetic field characteristics for the known magnetic stars: Mimes TC targets ** We need the wind characteristics: IUE archive UV spectrum ** Input in simulations of wind-field interaction, to determine the magnetospheric structure ** From the magnetosphere structure: infer observables such as X-ray emission, and confront with observations * X-ray properties of Mimes TC targets: those we have access to Chandra and XMM archives and those we have not ** New Chandra proposal David Cohen a écrit : > Hi, Veronique, > > I hope you're doing well. > I'm putting my talk for the Paris meeting together, and - I'm sure we have many things we should talk with each other about, but let's start with NU Ori and Par 1772. You are going to talk a little about them? Are you going to discuss their X-ray properties at all? > > I have Stan, Rich, Maurice, and Marc visiting on Friday. I'll be practicing my talk for them. > > David