collective effects of stellar winds and unidentified gamma-ray sources diego f. torres the...

Collective effects of stellar winds and unidentified gamma-ray sources

Diego F. Torres

THE MULTI-MESSENGER APPROACH TO UNIDENTIFIED GAMMA-RAY SOURCES

[email protected]

http://europa.ieec.fcr.es/english/recerca/map

Institut de Ciencies de l’Espai, Barcelona, Spain

Diego F. Torres dtorres@ ieec.uab.es http://europa.ieec.fcr.es/english/recerca/map

2 things discussed in the conference

Observational discovery of TeV sources without lower energy counterparts (not even GeV), e.g., Funk

Possibility of –at least two- explanations (Aharonian) Fastest diffusion of higher energy hadrons and follow-up interaction in

molecular clouds a bit separated from the accelerator Effect of stellar winds

(single stellar winds Torres et al. 2004, ApJ Letters) (collective stellar winds, this talk)


1999 – 2001 Observations 112 hr June - September 2002 Observations 137 hr

Unidentified TeV sources: The HEGRA source in Cygnus

Aharonian et al. 05


Making a long story short: no counterpart is identified

Butt et al. 05


Butt, et al. 2003

110 cataloged OB stars in Cyg OB2 shown as a surface density plot (stars per 4 arcmin2). Note that many stars in Cyg OB2 remain uncataloged – the total number of OB stars alone is expected to be ~2600 (Knodlseder 2002). Although the extinction pattern towards Cyg OB2 may control the observed surface density of OB stars, our analysis assumes that the observed distribution of OB stars tracks the actual distribution. The thick contours show the location probability (successively, 50%, 68%, 95%, and 99%) of the non-variable EGRET source 3EG 2033+4118. The red circle outlines the extent of the TeV source.

Enhancement of stellar density at the HEGRA TeV source

Butt et al. 05


HESS J1303-631: first serendipitous discovery during monitoring of the PSR B1259. [A&A 439 (2005) 1013]

Still unidentified. Extended source.

In correspondence the OB association Cen OB1 that contain al least 20 O stars and 1 WR star.

Also no known counterpart at lower freq yet.

Unidentified TeV sources: HESS J1303

Aharonian et al. 05


Hadronic -rays from stellar associations: needed ingredients

High-energy accelerated particles (up to hundreds TeV)

Collective effects of strong stellar winds & SNe at the association core

supperbubble large scale shock CR acceleration

(see, e.g., Montmerle 1979; Casse & Paul 1980; Bykov & Fleishman 1992a,b; Bykov 2001)

Dense enough target medium

(for p-p interactions 0 ) Proposed target: collective winds of OB stars

near the acceleration region


Winds of massive OB stars: basic observational facts

O and B stars lose a significant fraction of their mass in stellar winds with terminal velocities of order 103 km s-1.

With mass loss rates as high as M ~ 510-7 – 110-5 M◦ yr-1 the density at the base of the wind can reach 10-12 g cm-3 (e.g., Lamers & Cassinelli 1999, Ch.2).

Such winds are also permeated by significant magnetic fields, that can reach mG or even higher at tenths of pc from the surface (Surface magnetic field B ~ 10-100 G, Donati et al. 2001, 2002)

.


Wind-wind interaction that takes place in a dense stellar cluster or sub-cluster.

The stars are assumed to be uniformly distributed within the outer radius of the cluster.

The material ejected from the stars goes through stellar wind shocks (drawn as circles) and then participates in an outward flow [with mean velocity V(R) and density n(R)] which eventually leaves the cluster and interacts with the surrounding interstellar matter.

The gas within a collective wind


Istropic distribution of stars

Each with a wind velocity and mass loss rate

Mass and momentum conservation

Within the cluster

Ouside the cluster

Algebra + initial and boundary conditions

Numerically solvable set of equations

The gas within a collective windFrom Domingo-Santamaria & Torres, Hadronic Processes in Winds A&A, 448, 613, 2006


The gas within a collective wind

Wind Velocity Wind VelocityWind Density Wind Density

From Domingo-Santamaria & Torres, Hadronic Processes in Winds A&A, 448, 613, 2006


Penetrating the wind: diffusion vs. convection

Approach: comparison between timescales

Diffusion - Convection

Dr

td23 rV

rtc

3

rRVVEBft

trotp

c

d ,,,,, If >1 Convection dominates

If <1 Diffusion dominates

=1 defines Epmin for protons to penetrate the wind

E.g. only protons with energies

will enter into the wind whereas lower energy protons will be convected away wind modulation effect

MeV-GeV -ray emission substantially reduced


Torres, ApJ, 617,

966 (2004)

Numerical modeling of gamma-ray production, diffusion, losses


Effect of modulation

Knock-on emissivity positron emissivity Losses



Effect of modulation II

Differential Flux Integral Flux Opacity



Effect of modulation III

X=not detectable √=detectable


Concluding remarks

Collective effects of winds, related perhaps with unidentified TeV sources?

We have studied collective wind configurations produced by a number of massive stars, and obtained densities and expansion velocities of the stellar wind gas that is to be target for hadronic interactions in several examples.We have computed secondary particle production, electrons and positrons from charged pion decay, electrons from knock-on interactions, and solve the appropriate diffusion-loss equation with ionization, synchrotron, bremsstrahlung, inverse Compton, and expansion losses to obtain expected γ-ray emission from these regions, including in an approximate way the effect of cosmic ray modulation. Examples where different stellar configurations can produce sources for GLAST satellite, and theMAGIC/HESS/VERITAS telescopes in non-uniform ways, i.e., with or without the corresponding counterparts were shown.

Combined observations with GLAST and MAGIC/HESS will tell.

Most models produce no source at GeV energies, even for GLAST

Good candidates: Cygnus OB2 neighbourhood, Westerlund 1, HESS J1303-631

collective effects of stellar winds and unidentified gamma-ray sources diego f. torres the...

Documents

collective winds of

cataloged ob stars

o stars

esenglishrecercamap

total number of ob stars

stellar associations

talk slide

spain slide